Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Thu, Apr 19, 2018 at 3:44 AM, Jan Kara wrote:
> On Fri 13-04-18 15:03:51, Dan Williams wrote:
>> On Mon, Apr 9, 2018 at 9:51 AM, Dan Williams
>> wrote:
>> > On Mon, Apr 9, 2018 at 9:49 AM, Jan Kara wrote:
>> >> On Sat 07-04-18 12:38:24, Dan Williams wrote:
>> > [..]
>> >>> I wonder if this can be trivially solved by using srcu. I.e. we don't
>> >>> need to wait for a global quiescent state, just a
>> >>> get_user_pages_fast() quiescent state. ...or is that an abuse of the
>> >>> srcu api?
>> >>
>> >> Well, I'd rather use the percpu rwsemaphore (linux/percpu-rwsem.h) than
>> >> SRCU. It is a more-or-less standard locking mechanism rather than relying
>> >> on implementation properties of SRCU which is a data structure protection
>> >> method. And the overhead of percpu rwsemaphore for your use case should be
>> >> about the same as that of SRCU.
>> >
>> > I was just about to ask that. Yes, it seems they would share similar
>> > properties and it would be better to use the explicit implementation
>> > rather than a side effect of srcu.
>>
>> ...unfortunately:
>>
>> BUG: sleeping function called from invalid context at
>> ./include/linux/percpu-rwsem.h:34
>> [..]
>> Call Trace:
>> dump_stack+0x85/0xcb
>> ___might_sleep+0x15b/0x240
>> dax_layout_lock+0x18/0x80
>> get_user_pages_fast+0xf8/0x140
>>
>> ...and thinking about it more srcu is a better fit. We don't need the
>> 100% exclusion provided by an rwsem we only need the guarantee that
>> all cpus that might have been running get_user_pages_fast() have
>> finished it at least once.
>>
>> In my tests synchronize_srcu is a bit slower than unpatched for the
>> trivial 100 truncate test, but certainly not the 200x latency you were
>> seeing with syncrhonize_rcu.
>>
>> Elapsed time:
>> 0.006149178 unpatched
>> 0.009426360 srcu
>
> Hum, right. Yesterday I was looking into KSM for a different reason and
> I've noticed it also does writeprotect pages and deals with races with GUP.
> And what KSM relies on is:
>
> write_protect_page()
> ...
> entry = ptep_clear_flush(vma, pvmw.address, pvmw.pte);
> /*
>* Check that no O_DIRECT or similar I/O is in progress on the
>* page
>*/
> if (page_mapcount(page) + 1 + swapped != page_count(page)) {
> page used -> bail
Slick.
> }
>
> And this really works because gup_pte_range() does:
>
> page = pte_page(pte);
> head = compound_head(page);
>
> if (!page_cache_get_speculative(head))
> goto pte_unmap;
>
> if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> bail
Need to add a similar check to __gup_device_huge_pmd.
> }
>
> So either write_protect_page() page sees the elevated reference or
> gup_pte_range() bails because it will see the pte changed.
>
> In the truncate path things are a bit different but in principle the same
> should work - once truncate blocks page faults and unmaps pages from page
> tables, we can be sure GUP will not grab the page anymore or we'll see
> elevated page count. So IMO there's no need for any additional locking
> against the GUP path (but a comment explaining this is highly desirable I
> guess).
Yes, those "pte_val(pte) != pte_val(*ptep)" checks should be
documented for the same reason we require comments on rmb/wmb pairs.
I'll take a look, thanks Jan.
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Fri 13-04-18 15:03:51, Dan Williams wrote:
> On Mon, Apr 9, 2018 at 9:51 AM, Dan Williams wrote:
> > On Mon, Apr 9, 2018 at 9:49 AM, Jan Kara wrote:
> >> On Sat 07-04-18 12:38:24, Dan Williams wrote:
> > [..]
> >>> I wonder if this can be trivially solved by using srcu. I.e. we don't
> >>> need to wait for a global quiescent state, just a
> >>> get_user_pages_fast() quiescent state. ...or is that an abuse of the
> >>> srcu api?
> >>
> >> Well, I'd rather use the percpu rwsemaphore (linux/percpu-rwsem.h) than
> >> SRCU. It is a more-or-less standard locking mechanism rather than relying
> >> on implementation properties of SRCU which is a data structure protection
> >> method. And the overhead of percpu rwsemaphore for your use case should be
> >> about the same as that of SRCU.
> >
> > I was just about to ask that. Yes, it seems they would share similar
> > properties and it would be better to use the explicit implementation
> > rather than a side effect of srcu.
>
> ...unfortunately:
>
> BUG: sleeping function called from invalid context at
> ./include/linux/percpu-rwsem.h:34
> [..]
> Call Trace:
> dump_stack+0x85/0xcb
> ___might_sleep+0x15b/0x240
> dax_layout_lock+0x18/0x80
> get_user_pages_fast+0xf8/0x140
>
> ...and thinking about it more srcu is a better fit. We don't need the
> 100% exclusion provided by an rwsem we only need the guarantee that
> all cpus that might have been running get_user_pages_fast() have
> finished it at least once.
>
> In my tests synchronize_srcu is a bit slower than unpatched for the
> trivial 100 truncate test, but certainly not the 200x latency you were
> seeing with syncrhonize_rcu.
>
> Elapsed time:
> 0.006149178 unpatched
> 0.009426360 srcu
Hum, right. Yesterday I was looking into KSM for a different reason and
I've noticed it also does writeprotect pages and deals with races with GUP.
And what KSM relies on is:
write_protect_page()
...
entry = ptep_clear_flush(vma, pvmw.address, pvmw.pte);
/*
* Check that no O_DIRECT or similar I/O is in progress on the
* page
*/
if (page_mapcount(page) + 1 + swapped != page_count(page)) {
page used -> bail
}
And this really works because gup_pte_range() does:
page = pte_page(pte);
head = compound_head(page);
if (!page_cache_get_speculative(head))
goto pte_unmap;
if (unlikely(pte_val(pte) != pte_val(*ptep))) {
bail
}
So either write_protect_page() page sees the elevated reference or
gup_pte_range() bails because it will see the pte changed.
In the truncate path things are a bit different but in principle the same
should work - once truncate blocks page faults and unmaps pages from page
tables, we can be sure GUP will not grab the page anymore or we'll see
elevated page count. So IMO there's no need for any additional locking
against the GUP path (but a comment explaining this is highly desirable I
guess).
Honza
--
Jan Kara
SUSE Labs, CR
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Fri, Apr 13, 2018 at 03:03:51PM -0700, Dan Williams wrote: > On Mon, Apr 9, 2018 at 9:51 AM, Dan Williams wrote: > > On Mon, Apr 9, 2018 at 9:49 AM, Jan Kara wrote: > >> On Sat 07-04-18 12:38:24, Dan Williams wrote: > > [..] > >>> I wonder if this can be trivially solved by using srcu. I.e. we don't > >>> need to wait for a global quiescent state, just a > >>> get_user_pages_fast() quiescent state. ...or is that an abuse of the > >>> srcu api? > >> > >> Well, I'd rather use the percpu rwsemaphore (linux/percpu-rwsem.h) than > >> SRCU. It is a more-or-less standard locking mechanism rather than relying > >> on implementation properties of SRCU which is a data structure protection > >> method. And the overhead of percpu rwsemaphore for your use case should be > >> about the same as that of SRCU. > > > > I was just about to ask that. Yes, it seems they would share similar > > properties and it would be better to use the explicit implementation > > rather than a side effect of srcu. > > ...unfortunately: > > BUG: sleeping function called from invalid context at > ./include/linux/percpu-rwsem.h:34 > [..] > Call Trace: > dump_stack+0x85/0xcb > ___might_sleep+0x15b/0x240 > dax_layout_lock+0x18/0x80 > get_user_pages_fast+0xf8/0x140 > > ...and thinking about it more srcu is a better fit. We don't need the > 100% exclusion provided by an rwsem we only need the guarantee that > all cpus that might have been running get_user_pages_fast() have > finished it at least once. > > In my tests synchronize_srcu is a bit slower than unpatched for the > trivial 100 truncate test, but certainly not the 200x latency you were > seeing with syncrhonize_rcu. > > Elapsed time: > 0.006149178 unpatched > 0.009426360 srcu You might want to try synchronize_srcu_expedited(). Unlike plain RCU, it does not send IPIs, so should be less controversial. And it might well more than make up the performance difference you are seeing above. Thanx, Paul
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Mon, Apr 9, 2018 at 9:51 AM, Dan Williams wrote: > On Mon, Apr 9, 2018 at 9:49 AM, Jan Kara wrote: >> On Sat 07-04-18 12:38:24, Dan Williams wrote: > [..] >>> I wonder if this can be trivially solved by using srcu. I.e. we don't >>> need to wait for a global quiescent state, just a >>> get_user_pages_fast() quiescent state. ...or is that an abuse of the >>> srcu api? >> >> Well, I'd rather use the percpu rwsemaphore (linux/percpu-rwsem.h) than >> SRCU. It is a more-or-less standard locking mechanism rather than relying >> on implementation properties of SRCU which is a data structure protection >> method. And the overhead of percpu rwsemaphore for your use case should be >> about the same as that of SRCU. > > I was just about to ask that. Yes, it seems they would share similar > properties and it would be better to use the explicit implementation > rather than a side effect of srcu. ...unfortunately: BUG: sleeping function called from invalid context at ./include/linux/percpu-rwsem.h:34 [..] Call Trace: dump_stack+0x85/0xcb ___might_sleep+0x15b/0x240 dax_layout_lock+0x18/0x80 get_user_pages_fast+0xf8/0x140 ...and thinking about it more srcu is a better fit. We don't need the 100% exclusion provided by an rwsem we only need the guarantee that all cpus that might have been running get_user_pages_fast() have finished it at least once. In my tests synchronize_srcu is a bit slower than unpatched for the trivial 100 truncate test, but certainly not the 200x latency you were seeing with syncrhonize_rcu. Elapsed time: 0.006149178 unpatched 0.009426360 srcu
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Mon, Apr 09, 2018 at 06:39:10PM +0200, Jan Kara wrote:
> On Sat 07-04-18 20:11:13, Paul E. McKenney wrote:
> > On Sat, Apr 07, 2018 at 12:38:24PM -0700, Dan Williams wrote:
> > > [ adding Paul and Josh ]
> > >
> > > On Wed, Apr 4, 2018 at 2:46 AM, Jan Kara wrote:
> > > > On Fri 30-03-18 21:03:30, Dan Williams wrote:
> > > >> Background:
> > > >>
> > > >> get_user_pages() in the filesystem pins file backed memory pages for
> > > >> access by devices performing dma. However, it only pins the memory
> > > >> pages
> > > >> not the page-to-file offset association. If a file is truncated the
> > > >> pages are mapped out of the file and dma may continue indefinitely into
> > > >> a page that is owned by a device driver. This breaks coherency of the
> > > >> file vs dma, but the assumption is that if userspace wants the
> > > >> file-space truncated it does not matter what data is inbound from the
> > > >> device, it is not relevant anymore. The only expectation is that dma
> > > >> can
> > > >> safely continue while the filesystem reallocates the block(s).
> > > >>
> > > >> Problem:
> > > >>
> > > >> This expectation that dma can safely continue while the filesystem
> > > >> changes the block map is broken by dax. With dax the target dma page
> > > >> *is* the filesystem block. The model of leaving the page pinned for
> > > >> dma,
> > > >> but truncating the file block out of the file, means that the filesytem
> > > >> is free to reallocate a block under active dma to another file and now
> > > >> the expected data-incoherency situation has turned into active
> > > >> data-corruption.
> > > >>
> > > >> Solution:
> > > >>
> > > >> Defer all filesystem operations (fallocate(), truncate()) on a dax mode
> > > >> file while any page/block in the file is under active dma. This
> > > >> solution
> > > >> assumes that dma is transient. Cases where dma operations are known to
> > > >> not be transient, like RDMA, have been explicitly disabled via
> > > >> commits like 5f1d43de5416 "IB/core: disable memory registration of
> > > >> filesystem-dax vmas".
> > > >>
> > > >> The dax_layout_busy_page() routine is called by filesystems with a lock
> > > >> held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
> > > >> The process of looking up a busy page invalidates all mappings
> > > >> to trigger any subsequent get_user_pages() to block on i_mmap_lock.
> > > >> The filesystem continues to call dax_layout_busy_page() until it
> > > >> finally
> > > >> returns no more active pages. This approach assumes that the page
> > > >> pinning is transient, if that assumption is violated the system would
> > > >> have likely hung from the uncompleted I/O.
> > > >>
> > > >> Cc: Jan Kara
> > > >> Cc: Jeff Moyer
> > > >> Cc: Dave Chinner
> > > >> Cc: Matthew Wilcox
> > > >> Cc: Alexander Viro
> > > >> Cc: "Darrick J. Wong"
> > > >> Cc: Ross Zwisler
> > > >> Cc: Dave Hansen
> > > >> Cc: Andrew Morton
> > > >> Reported-by: Christoph Hellwig
> > > >> Reviewed-by: Christoph Hellwig
> > > >> Signed-off-by: Dan Williams
> > > >> ---
> > > >> drivers/dax/super.c |2 +
> > > >> fs/dax.c| 92
> > > >> +++
> > > >> include/linux/dax.h | 25 ++
> > > >> mm/gup.c|5 +++
> > > >> 4 files changed, 123 insertions(+), 1 deletion(-)
> > > >
> > > > ...
> > > >
> > > >> +/**
> > > >> + * dax_layout_busy_page - find first pinned page in @mapping
> > > >> + * @mapping: address space to scan for a page with ref count > 1
> > > >> + *
> > > >> + * DAX requires ZONE_DEVICE mapped pages. These pages are never
> > > >> + * 'onlined' to the page allocator so they are considered idle when
> > > >> + * page->count == 1. A filesystem uses this interface to determine if
> > > >> + * any page in the mapping is busy, i.e. for DMA, or other
> > > >> + * get_user_pages() usages.
> > > >> + *
> > > >> + * It is expected that the filesystem is holding locks to block the
> > > >> + * establishment of new mappings in this address_space. I.e. it
> > > >> expects
> > > >> + * to be able to run unmap_mapping_range() and subsequently not race
> > > >> + * mapping_mapped() becoming true. It expects that get_user_pages()
> > > >> pte
> > > >> + * walks are performed under rcu_read_lock().
> > > >> + */
> > > >> +struct page *dax_layout_busy_page(struct address_space *mapping)
> > > >> +{
> > > >> + pgoff_t indices[PAGEVEC_SIZE];
> > > >> + struct page *page = NULL;
> > > >> + struct pagevec pvec;
> > > >> + pgoff_t index, end;
> > > >> + unsigned i;
> > > >> +
> > > >> + /*
> > > >> + * In the 'limited' case get_user_pages() for dax is disabled.
> > > >> + */
> > > >> + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> > > >> + return NULL;
> > > >> +
> > > >> + if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> > > >> + return NULL;
> > > >> +
> > > >> + pagevec_init(&pvec);
> >
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Mon, Apr 9, 2018 at 9:49 AM, Jan Kara wrote: > On Sat 07-04-18 12:38:24, Dan Williams wrote: [..] >> I wonder if this can be trivially solved by using srcu. I.e. we don't >> need to wait for a global quiescent state, just a >> get_user_pages_fast() quiescent state. ...or is that an abuse of the >> srcu api? > > Well, I'd rather use the percpu rwsemaphore (linux/percpu-rwsem.h) than > SRCU. It is a more-or-less standard locking mechanism rather than relying > on implementation properties of SRCU which is a data structure protection > method. And the overhead of percpu rwsemaphore for your use case should be > about the same as that of SRCU. I was just about to ask that. Yes, it seems they would share similar properties and it would be better to use the explicit implementation rather than a side effect of srcu.
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Sat 07-04-18 12:38:24, Dan Williams wrote:
> [ adding Paul and Josh ]
>
> On Wed, Apr 4, 2018 at 2:46 AM, Jan Kara wrote:
> > On Fri 30-03-18 21:03:30, Dan Williams wrote:
> >> Background:
> >>
> >> get_user_pages() in the filesystem pins file backed memory pages for
> >> access by devices performing dma. However, it only pins the memory pages
> >> not the page-to-file offset association. If a file is truncated the
> >> pages are mapped out of the file and dma may continue indefinitely into
> >> a page that is owned by a device driver. This breaks coherency of the
> >> file vs dma, but the assumption is that if userspace wants the
> >> file-space truncated it does not matter what data is inbound from the
> >> device, it is not relevant anymore. The only expectation is that dma can
> >> safely continue while the filesystem reallocates the block(s).
> >>
> >> Problem:
> >>
> >> This expectation that dma can safely continue while the filesystem
> >> changes the block map is broken by dax. With dax the target dma page
> >> *is* the filesystem block. The model of leaving the page pinned for dma,
> >> but truncating the file block out of the file, means that the filesytem
> >> is free to reallocate a block under active dma to another file and now
> >> the expected data-incoherency situation has turned into active
> >> data-corruption.
> >>
> >> Solution:
> >>
> >> Defer all filesystem operations (fallocate(), truncate()) on a dax mode
> >> file while any page/block in the file is under active dma. This solution
> >> assumes that dma is transient. Cases where dma operations are known to
> >> not be transient, like RDMA, have been explicitly disabled via
> >> commits like 5f1d43de5416 "IB/core: disable memory registration of
> >> filesystem-dax vmas".
> >>
> >> The dax_layout_busy_page() routine is called by filesystems with a lock
> >> held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
> >> The process of looking up a busy page invalidates all mappings
> >> to trigger any subsequent get_user_pages() to block on i_mmap_lock.
> >> The filesystem continues to call dax_layout_busy_page() until it finally
> >> returns no more active pages. This approach assumes that the page
> >> pinning is transient, if that assumption is violated the system would
> >> have likely hung from the uncompleted I/O.
> >>
> >> Cc: Jan Kara
> >> Cc: Jeff Moyer
> >> Cc: Dave Chinner
> >> Cc: Matthew Wilcox
> >> Cc: Alexander Viro
> >> Cc: "Darrick J. Wong"
> >> Cc: Ross Zwisler
> >> Cc: Dave Hansen
> >> Cc: Andrew Morton
> >> Reported-by: Christoph Hellwig
> >> Reviewed-by: Christoph Hellwig
> >> Signed-off-by: Dan Williams
> >> ---
> >> drivers/dax/super.c |2 +
> >> fs/dax.c| 92
> >> +++
> >> include/linux/dax.h | 25 ++
> >> mm/gup.c|5 +++
> >> 4 files changed, 123 insertions(+), 1 deletion(-)
> >
> > ...
> >
> >> +/**
> >> + * dax_layout_busy_page - find first pinned page in @mapping
> >> + * @mapping: address space to scan for a page with ref count > 1
> >> + *
> >> + * DAX requires ZONE_DEVICE mapped pages. These pages are never
> >> + * 'onlined' to the page allocator so they are considered idle when
> >> + * page->count == 1. A filesystem uses this interface to determine if
> >> + * any page in the mapping is busy, i.e. for DMA, or other
> >> + * get_user_pages() usages.
> >> + *
> >> + * It is expected that the filesystem is holding locks to block the
> >> + * establishment of new mappings in this address_space. I.e. it expects
> >> + * to be able to run unmap_mapping_range() and subsequently not race
> >> + * mapping_mapped() becoming true. It expects that get_user_pages() pte
> >> + * walks are performed under rcu_read_lock().
> >> + */
> >> +struct page *dax_layout_busy_page(struct address_space *mapping)
> >> +{
> >> + pgoff_t indices[PAGEVEC_SIZE];
> >> + struct page *page = NULL;
> >> + struct pagevec pvec;
> >> + pgoff_t index, end;
> >> + unsigned i;
> >> +
> >> + /*
> >> + * In the 'limited' case get_user_pages() for dax is disabled.
> >> + */
> >> + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> >> + return NULL;
> >> +
> >> + if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> >> + return NULL;
> >> +
> >> + pagevec_init(&pvec);
> >> + index = 0;
> >> + end = -1;
> >> + /*
> >> + * Flush dax_layout_lock() sections to ensure all possible page
> >> + * references have been taken, or otherwise arrange for faults
> >> + * to block on the filesystem lock that is taken for
> >> + * establishing new mappings.
> >> + */
> >> + unmap_mapping_range(mapping, 0, 0, 1);
> >> + synchronize_rcu();
> >
> > So I still don't like the use of RCU for this. It just seems as an abuse to
> > use RCU like that. Furthermore it has a hefty latency cost for the truncate
> > path. A trivial test to trunc
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Sat 07-04-18 20:11:13, Paul E. McKenney wrote:
> On Sat, Apr 07, 2018 at 12:38:24PM -0700, Dan Williams wrote:
> > [ adding Paul and Josh ]
> >
> > On Wed, Apr 4, 2018 at 2:46 AM, Jan Kara wrote:
> > > On Fri 30-03-18 21:03:30, Dan Williams wrote:
> > >> Background:
> > >>
> > >> get_user_pages() in the filesystem pins file backed memory pages for
> > >> access by devices performing dma. However, it only pins the memory pages
> > >> not the page-to-file offset association. If a file is truncated the
> > >> pages are mapped out of the file and dma may continue indefinitely into
> > >> a page that is owned by a device driver. This breaks coherency of the
> > >> file vs dma, but the assumption is that if userspace wants the
> > >> file-space truncated it does not matter what data is inbound from the
> > >> device, it is not relevant anymore. The only expectation is that dma can
> > >> safely continue while the filesystem reallocates the block(s).
> > >>
> > >> Problem:
> > >>
> > >> This expectation that dma can safely continue while the filesystem
> > >> changes the block map is broken by dax. With dax the target dma page
> > >> *is* the filesystem block. The model of leaving the page pinned for dma,
> > >> but truncating the file block out of the file, means that the filesytem
> > >> is free to reallocate a block under active dma to another file and now
> > >> the expected data-incoherency situation has turned into active
> > >> data-corruption.
> > >>
> > >> Solution:
> > >>
> > >> Defer all filesystem operations (fallocate(), truncate()) on a dax mode
> > >> file while any page/block in the file is under active dma. This solution
> > >> assumes that dma is transient. Cases where dma operations are known to
> > >> not be transient, like RDMA, have been explicitly disabled via
> > >> commits like 5f1d43de5416 "IB/core: disable memory registration of
> > >> filesystem-dax vmas".
> > >>
> > >> The dax_layout_busy_page() routine is called by filesystems with a lock
> > >> held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
> > >> The process of looking up a busy page invalidates all mappings
> > >> to trigger any subsequent get_user_pages() to block on i_mmap_lock.
> > >> The filesystem continues to call dax_layout_busy_page() until it finally
> > >> returns no more active pages. This approach assumes that the page
> > >> pinning is transient, if that assumption is violated the system would
> > >> have likely hung from the uncompleted I/O.
> > >>
> > >> Cc: Jan Kara
> > >> Cc: Jeff Moyer
> > >> Cc: Dave Chinner
> > >> Cc: Matthew Wilcox
> > >> Cc: Alexander Viro
> > >> Cc: "Darrick J. Wong"
> > >> Cc: Ross Zwisler
> > >> Cc: Dave Hansen
> > >> Cc: Andrew Morton
> > >> Reported-by: Christoph Hellwig
> > >> Reviewed-by: Christoph Hellwig
> > >> Signed-off-by: Dan Williams
> > >> ---
> > >> drivers/dax/super.c |2 +
> > >> fs/dax.c| 92
> > >> +++
> > >> include/linux/dax.h | 25 ++
> > >> mm/gup.c|5 +++
> > >> 4 files changed, 123 insertions(+), 1 deletion(-)
> > >
> > > ...
> > >
> > >> +/**
> > >> + * dax_layout_busy_page - find first pinned page in @mapping
> > >> + * @mapping: address space to scan for a page with ref count > 1
> > >> + *
> > >> + * DAX requires ZONE_DEVICE mapped pages. These pages are never
> > >> + * 'onlined' to the page allocator so they are considered idle when
> > >> + * page->count == 1. A filesystem uses this interface to determine if
> > >> + * any page in the mapping is busy, i.e. for DMA, or other
> > >> + * get_user_pages() usages.
> > >> + *
> > >> + * It is expected that the filesystem is holding locks to block the
> > >> + * establishment of new mappings in this address_space. I.e. it expects
> > >> + * to be able to run unmap_mapping_range() and subsequently not race
> > >> + * mapping_mapped() becoming true. It expects that get_user_pages() pte
> > >> + * walks are performed under rcu_read_lock().
> > >> + */
> > >> +struct page *dax_layout_busy_page(struct address_space *mapping)
> > >> +{
> > >> + pgoff_t indices[PAGEVEC_SIZE];
> > >> + struct page *page = NULL;
> > >> + struct pagevec pvec;
> > >> + pgoff_t index, end;
> > >> + unsigned i;
> > >> +
> > >> + /*
> > >> + * In the 'limited' case get_user_pages() for dax is disabled.
> > >> + */
> > >> + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> > >> + return NULL;
> > >> +
> > >> + if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> > >> + return NULL;
> > >> +
> > >> + pagevec_init(&pvec);
> > >> + index = 0;
> > >> + end = -1;
> > >> + /*
> > >> + * Flush dax_layout_lock() sections to ensure all possible page
> > >> + * references have been taken, or otherwise arrange for faults
> > >> + * to block on the filesystem lock that is taken for
> > >> + * establishing new mappings.
> > >> +
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Sat, Apr 07, 2018 at 12:38:24PM -0700, Dan Williams wrote:
> [ adding Paul and Josh ]
>
> On Wed, Apr 4, 2018 at 2:46 AM, Jan Kara wrote:
> > On Fri 30-03-18 21:03:30, Dan Williams wrote:
> >> Background:
> >>
> >> get_user_pages() in the filesystem pins file backed memory pages for
> >> access by devices performing dma. However, it only pins the memory pages
> >> not the page-to-file offset association. If a file is truncated the
> >> pages are mapped out of the file and dma may continue indefinitely into
> >> a page that is owned by a device driver. This breaks coherency of the
> >> file vs dma, but the assumption is that if userspace wants the
> >> file-space truncated it does not matter what data is inbound from the
> >> device, it is not relevant anymore. The only expectation is that dma can
> >> safely continue while the filesystem reallocates the block(s).
> >>
> >> Problem:
> >>
> >> This expectation that dma can safely continue while the filesystem
> >> changes the block map is broken by dax. With dax the target dma page
> >> *is* the filesystem block. The model of leaving the page pinned for dma,
> >> but truncating the file block out of the file, means that the filesytem
> >> is free to reallocate a block under active dma to another file and now
> >> the expected data-incoherency situation has turned into active
> >> data-corruption.
> >>
> >> Solution:
> >>
> >> Defer all filesystem operations (fallocate(), truncate()) on a dax mode
> >> file while any page/block in the file is under active dma. This solution
> >> assumes that dma is transient. Cases where dma operations are known to
> >> not be transient, like RDMA, have been explicitly disabled via
> >> commits like 5f1d43de5416 "IB/core: disable memory registration of
> >> filesystem-dax vmas".
> >>
> >> The dax_layout_busy_page() routine is called by filesystems with a lock
> >> held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
> >> The process of looking up a busy page invalidates all mappings
> >> to trigger any subsequent get_user_pages() to block on i_mmap_lock.
> >> The filesystem continues to call dax_layout_busy_page() until it finally
> >> returns no more active pages. This approach assumes that the page
> >> pinning is transient, if that assumption is violated the system would
> >> have likely hung from the uncompleted I/O.
> >>
> >> Cc: Jan Kara
> >> Cc: Jeff Moyer
> >> Cc: Dave Chinner
> >> Cc: Matthew Wilcox
> >> Cc: Alexander Viro
> >> Cc: "Darrick J. Wong"
> >> Cc: Ross Zwisler
> >> Cc: Dave Hansen
> >> Cc: Andrew Morton
> >> Reported-by: Christoph Hellwig
> >> Reviewed-by: Christoph Hellwig
> >> Signed-off-by: Dan Williams
> >> ---
> >> drivers/dax/super.c |2 +
> >> fs/dax.c| 92
> >> +++
> >> include/linux/dax.h | 25 ++
> >> mm/gup.c|5 +++
> >> 4 files changed, 123 insertions(+), 1 deletion(-)
> >
> > ...
> >
> >> +/**
> >> + * dax_layout_busy_page - find first pinned page in @mapping
> >> + * @mapping: address space to scan for a page with ref count > 1
> >> + *
> >> + * DAX requires ZONE_DEVICE mapped pages. These pages are never
> >> + * 'onlined' to the page allocator so they are considered idle when
> >> + * page->count == 1. A filesystem uses this interface to determine if
> >> + * any page in the mapping is busy, i.e. for DMA, or other
> >> + * get_user_pages() usages.
> >> + *
> >> + * It is expected that the filesystem is holding locks to block the
> >> + * establishment of new mappings in this address_space. I.e. it expects
> >> + * to be able to run unmap_mapping_range() and subsequently not race
> >> + * mapping_mapped() becoming true. It expects that get_user_pages() pte
> >> + * walks are performed under rcu_read_lock().
> >> + */
> >> +struct page *dax_layout_busy_page(struct address_space *mapping)
> >> +{
> >> + pgoff_t indices[PAGEVEC_SIZE];
> >> + struct page *page = NULL;
> >> + struct pagevec pvec;
> >> + pgoff_t index, end;
> >> + unsigned i;
> >> +
> >> + /*
> >> + * In the 'limited' case get_user_pages() for dax is disabled.
> >> + */
> >> + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> >> + return NULL;
> >> +
> >> + if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> >> + return NULL;
> >> +
> >> + pagevec_init(&pvec);
> >> + index = 0;
> >> + end = -1;
> >> + /*
> >> + * Flush dax_layout_lock() sections to ensure all possible page
> >> + * references have been taken, or otherwise arrange for faults
> >> + * to block on the filesystem lock that is taken for
> >> + * establishing new mappings.
> >> + */
> >> + unmap_mapping_range(mapping, 0, 0, 1);
> >> + synchronize_rcu();
> >
> > So I still don't like the use of RCU for this. It just seems as an abuse to
> > use RCU like that. Furthermore it has a hefty latency cost for the truncate
> > path. A trivi
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
[ adding Paul and Josh ]
On Wed, Apr 4, 2018 at 2:46 AM, Jan Kara wrote:
> On Fri 30-03-18 21:03:30, Dan Williams wrote:
>> Background:
>>
>> get_user_pages() in the filesystem pins file backed memory pages for
>> access by devices performing dma. However, it only pins the memory pages
>> not the page-to-file offset association. If a file is truncated the
>> pages are mapped out of the file and dma may continue indefinitely into
>> a page that is owned by a device driver. This breaks coherency of the
>> file vs dma, but the assumption is that if userspace wants the
>> file-space truncated it does not matter what data is inbound from the
>> device, it is not relevant anymore. The only expectation is that dma can
>> safely continue while the filesystem reallocates the block(s).
>>
>> Problem:
>>
>> This expectation that dma can safely continue while the filesystem
>> changes the block map is broken by dax. With dax the target dma page
>> *is* the filesystem block. The model of leaving the page pinned for dma,
>> but truncating the file block out of the file, means that the filesytem
>> is free to reallocate a block under active dma to another file and now
>> the expected data-incoherency situation has turned into active
>> data-corruption.
>>
>> Solution:
>>
>> Defer all filesystem operations (fallocate(), truncate()) on a dax mode
>> file while any page/block in the file is under active dma. This solution
>> assumes that dma is transient. Cases where dma operations are known to
>> not be transient, like RDMA, have been explicitly disabled via
>> commits like 5f1d43de5416 "IB/core: disable memory registration of
>> filesystem-dax vmas".
>>
>> The dax_layout_busy_page() routine is called by filesystems with a lock
>> held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
>> The process of looking up a busy page invalidates all mappings
>> to trigger any subsequent get_user_pages() to block on i_mmap_lock.
>> The filesystem continues to call dax_layout_busy_page() until it finally
>> returns no more active pages. This approach assumes that the page
>> pinning is transient, if that assumption is violated the system would
>> have likely hung from the uncompleted I/O.
>>
>> Cc: Jan Kara
>> Cc: Jeff Moyer
>> Cc: Dave Chinner
>> Cc: Matthew Wilcox
>> Cc: Alexander Viro
>> Cc: "Darrick J. Wong"
>> Cc: Ross Zwisler
>> Cc: Dave Hansen
>> Cc: Andrew Morton
>> Reported-by: Christoph Hellwig
>> Reviewed-by: Christoph Hellwig
>> Signed-off-by: Dan Williams
>> ---
>> drivers/dax/super.c |2 +
>> fs/dax.c| 92
>> +++
>> include/linux/dax.h | 25 ++
>> mm/gup.c|5 +++
>> 4 files changed, 123 insertions(+), 1 deletion(-)
>
> ...
>
>> +/**
>> + * dax_layout_busy_page - find first pinned page in @mapping
>> + * @mapping: address space to scan for a page with ref count > 1
>> + *
>> + * DAX requires ZONE_DEVICE mapped pages. These pages are never
>> + * 'onlined' to the page allocator so they are considered idle when
>> + * page->count == 1. A filesystem uses this interface to determine if
>> + * any page in the mapping is busy, i.e. for DMA, or other
>> + * get_user_pages() usages.
>> + *
>> + * It is expected that the filesystem is holding locks to block the
>> + * establishment of new mappings in this address_space. I.e. it expects
>> + * to be able to run unmap_mapping_range() and subsequently not race
>> + * mapping_mapped() becoming true. It expects that get_user_pages() pte
>> + * walks are performed under rcu_read_lock().
>> + */
>> +struct page *dax_layout_busy_page(struct address_space *mapping)
>> +{
>> + pgoff_t indices[PAGEVEC_SIZE];
>> + struct page *page = NULL;
>> + struct pagevec pvec;
>> + pgoff_t index, end;
>> + unsigned i;
>> +
>> + /*
>> + * In the 'limited' case get_user_pages() for dax is disabled.
>> + */
>> + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
>> + return NULL;
>> +
>> + if (!dax_mapping(mapping) || !mapping_mapped(mapping))
>> + return NULL;
>> +
>> + pagevec_init(&pvec);
>> + index = 0;
>> + end = -1;
>> + /*
>> + * Flush dax_layout_lock() sections to ensure all possible page
>> + * references have been taken, or otherwise arrange for faults
>> + * to block on the filesystem lock that is taken for
>> + * establishing new mappings.
>> + */
>> + unmap_mapping_range(mapping, 0, 0, 1);
>> + synchronize_rcu();
>
> So I still don't like the use of RCU for this. It just seems as an abuse to
> use RCU like that. Furthermore it has a hefty latency cost for the truncate
> path. A trivial test to truncate 100 times the last page of a 16k file that
> is mmaped (only the first page):
>
> DAX+your patches3.899s
> non-DAX 0.015s
>
> So you can see synchronize_rcu() increased time to run truncate(2) more
> than 200 times (the process is indeed sitting
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Wed, Apr 4, 2018 at 2:46 AM, Jan Kara wrote:
> On Fri 30-03-18 21:03:30, Dan Williams wrote:
>> Background:
>>
>> get_user_pages() in the filesystem pins file backed memory pages for
>> access by devices performing dma. However, it only pins the memory pages
>> not the page-to-file offset association. If a file is truncated the
>> pages are mapped out of the file and dma may continue indefinitely into
>> a page that is owned by a device driver. This breaks coherency of the
>> file vs dma, but the assumption is that if userspace wants the
>> file-space truncated it does not matter what data is inbound from the
>> device, it is not relevant anymore. The only expectation is that dma can
>> safely continue while the filesystem reallocates the block(s).
>>
>> Problem:
>>
>> This expectation that dma can safely continue while the filesystem
>> changes the block map is broken by dax. With dax the target dma page
>> *is* the filesystem block. The model of leaving the page pinned for dma,
>> but truncating the file block out of the file, means that the filesytem
>> is free to reallocate a block under active dma to another file and now
>> the expected data-incoherency situation has turned into active
>> data-corruption.
>>
>> Solution:
>>
>> Defer all filesystem operations (fallocate(), truncate()) on a dax mode
>> file while any page/block in the file is under active dma. This solution
>> assumes that dma is transient. Cases where dma operations are known to
>> not be transient, like RDMA, have been explicitly disabled via
>> commits like 5f1d43de5416 "IB/core: disable memory registration of
>> filesystem-dax vmas".
>>
>> The dax_layout_busy_page() routine is called by filesystems with a lock
>> held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
>> The process of looking up a busy page invalidates all mappings
>> to trigger any subsequent get_user_pages() to block on i_mmap_lock.
>> The filesystem continues to call dax_layout_busy_page() until it finally
>> returns no more active pages. This approach assumes that the page
>> pinning is transient, if that assumption is violated the system would
>> have likely hung from the uncompleted I/O.
>>
>> Cc: Jan Kara
>> Cc: Jeff Moyer
>> Cc: Dave Chinner
>> Cc: Matthew Wilcox
>> Cc: Alexander Viro
>> Cc: "Darrick J. Wong"
>> Cc: Ross Zwisler
>> Cc: Dave Hansen
>> Cc: Andrew Morton
>> Reported-by: Christoph Hellwig
>> Reviewed-by: Christoph Hellwig
>> Signed-off-by: Dan Williams
>> ---
>> drivers/dax/super.c |2 +
>> fs/dax.c| 92
>> +++
>> include/linux/dax.h | 25 ++
>> mm/gup.c|5 +++
>> 4 files changed, 123 insertions(+), 1 deletion(-)
>
> ...
>
>> +/**
>> + * dax_layout_busy_page - find first pinned page in @mapping
>> + * @mapping: address space to scan for a page with ref count > 1
>> + *
>> + * DAX requires ZONE_DEVICE mapped pages. These pages are never
>> + * 'onlined' to the page allocator so they are considered idle when
>> + * page->count == 1. A filesystem uses this interface to determine if
>> + * any page in the mapping is busy, i.e. for DMA, or other
>> + * get_user_pages() usages.
>> + *
>> + * It is expected that the filesystem is holding locks to block the
>> + * establishment of new mappings in this address_space. I.e. it expects
>> + * to be able to run unmap_mapping_range() and subsequently not race
>> + * mapping_mapped() becoming true. It expects that get_user_pages() pte
>> + * walks are performed under rcu_read_lock().
>> + */
>> +struct page *dax_layout_busy_page(struct address_space *mapping)
>> +{
>> + pgoff_t indices[PAGEVEC_SIZE];
>> + struct page *page = NULL;
>> + struct pagevec pvec;
>> + pgoff_t index, end;
>> + unsigned i;
>> +
>> + /*
>> + * In the 'limited' case get_user_pages() for dax is disabled.
>> + */
>> + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
>> + return NULL;
>> +
>> + if (!dax_mapping(mapping) || !mapping_mapped(mapping))
>> + return NULL;
>> +
>> + pagevec_init(&pvec);
>> + index = 0;
>> + end = -1;
>> + /*
>> + * Flush dax_layout_lock() sections to ensure all possible page
>> + * references have been taken, or otherwise arrange for faults
>> + * to block on the filesystem lock that is taken for
>> + * establishing new mappings.
>> + */
>> + unmap_mapping_range(mapping, 0, 0, 1);
>> + synchronize_rcu();
>
> So I still don't like the use of RCU for this. It just seems as an abuse to
> use RCU like that. Furthermore it has a hefty latency cost for the truncate
> path. A trivial test to truncate 100 times the last page of a 16k file that
> is mmaped (only the first page):
>
> DAX+your patches3.899s
> non-DAX 0.015s
>
> So you can see synchronize_rcu() increased time to run truncate(2) more
> than 200 times (the process is indeed sitting in __wait_rcu_gp all the
>
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Wed 04-04-18 11:46:56, Jan Kara wrote:
> On Fri 30-03-18 21:03:30, Dan Williams wrote:
> > Background:
> >
> > get_user_pages() in the filesystem pins file backed memory pages for
> > access by devices performing dma. However, it only pins the memory pages
> > not the page-to-file offset association. If a file is truncated the
> > pages are mapped out of the file and dma may continue indefinitely into
> > a page that is owned by a device driver. This breaks coherency of the
> > file vs dma, but the assumption is that if userspace wants the
> > file-space truncated it does not matter what data is inbound from the
> > device, it is not relevant anymore. The only expectation is that dma can
> > safely continue while the filesystem reallocates the block(s).
> >
> > Problem:
> >
> > This expectation that dma can safely continue while the filesystem
> > changes the block map is broken by dax. With dax the target dma page
> > *is* the filesystem block. The model of leaving the page pinned for dma,
> > but truncating the file block out of the file, means that the filesytem
> > is free to reallocate a block under active dma to another file and now
> > the expected data-incoherency situation has turned into active
> > data-corruption.
> >
> > Solution:
> >
> > Defer all filesystem operations (fallocate(), truncate()) on a dax mode
> > file while any page/block in the file is under active dma. This solution
> > assumes that dma is transient. Cases where dma operations are known to
> > not be transient, like RDMA, have been explicitly disabled via
> > commits like 5f1d43de5416 "IB/core: disable memory registration of
> > filesystem-dax vmas".
> >
> > The dax_layout_busy_page() routine is called by filesystems with a lock
> > held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
> > The process of looking up a busy page invalidates all mappings
> > to trigger any subsequent get_user_pages() to block on i_mmap_lock.
> > The filesystem continues to call dax_layout_busy_page() until it finally
> > returns no more active pages. This approach assumes that the page
> > pinning is transient, if that assumption is violated the system would
> > have likely hung from the uncompleted I/O.
> >
> > Cc: Jan Kara
> > Cc: Jeff Moyer
> > Cc: Dave Chinner
> > Cc: Matthew Wilcox
> > Cc: Alexander Viro
> > Cc: "Darrick J. Wong"
> > Cc: Ross Zwisler
> > Cc: Dave Hansen
> > Cc: Andrew Morton
> > Reported-by: Christoph Hellwig
> > Reviewed-by: Christoph Hellwig
> > Signed-off-by: Dan Williams
> > ---
> > drivers/dax/super.c |2 +
> > fs/dax.c| 92
> > +++
> > include/linux/dax.h | 25 ++
> > mm/gup.c|5 +++
> > 4 files changed, 123 insertions(+), 1 deletion(-)
>
> ...
>
> > +/**
> > + * dax_layout_busy_page - find first pinned page in @mapping
> > + * @mapping: address space to scan for a page with ref count > 1
> > + *
> > + * DAX requires ZONE_DEVICE mapped pages. These pages are never
> > + * 'onlined' to the page allocator so they are considered idle when
> > + * page->count == 1. A filesystem uses this interface to determine if
> > + * any page in the mapping is busy, i.e. for DMA, or other
> > + * get_user_pages() usages.
> > + *
> > + * It is expected that the filesystem is holding locks to block the
> > + * establishment of new mappings in this address_space. I.e. it expects
> > + * to be able to run unmap_mapping_range() and subsequently not race
> > + * mapping_mapped() becoming true. It expects that get_user_pages() pte
> > + * walks are performed under rcu_read_lock().
> > + */
> > +struct page *dax_layout_busy_page(struct address_space *mapping)
> > +{
> > + pgoff_t indices[PAGEVEC_SIZE];
> > + struct page *page = NULL;
> > + struct pagevec pvec;
> > + pgoff_t index, end;
> > + unsigned i;
> > +
> > + /*
> > +* In the 'limited' case get_user_pages() for dax is disabled.
> > +*/
> > + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> > + return NULL;
> > +
> > + if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> > + return NULL;
> > +
> > + pagevec_init(&pvec);
> > + index = 0;
> > + end = -1;
> > + /*
> > +* Flush dax_layout_lock() sections to ensure all possible page
> > +* references have been taken, or otherwise arrange for faults
> > +* to block on the filesystem lock that is taken for
> > +* establishing new mappings.
> > +*/
> > + unmap_mapping_range(mapping, 0, 0, 1);
> > + synchronize_rcu();
>
> So I still don't like the use of RCU for this. It just seems as an abuse to
> use RCU like that. Furthermore it has a hefty latency cost for the truncate
> path. A trivial test to truncate 100 times the last page of a 16k file that
> is mmaped (only the first page):
>
> DAX+your patches 3.899s
> non-DAX 0.015s
>
> So you can see synchronize_rcu() increased time to run truncate(2) more
> than 2
Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings
On Fri 30-03-18 21:03:30, Dan Williams wrote:
> Background:
>
> get_user_pages() in the filesystem pins file backed memory pages for
> access by devices performing dma. However, it only pins the memory pages
> not the page-to-file offset association. If a file is truncated the
> pages are mapped out of the file and dma may continue indefinitely into
> a page that is owned by a device driver. This breaks coherency of the
> file vs dma, but the assumption is that if userspace wants the
> file-space truncated it does not matter what data is inbound from the
> device, it is not relevant anymore. The only expectation is that dma can
> safely continue while the filesystem reallocates the block(s).
>
> Problem:
>
> This expectation that dma can safely continue while the filesystem
> changes the block map is broken by dax. With dax the target dma page
> *is* the filesystem block. The model of leaving the page pinned for dma,
> but truncating the file block out of the file, means that the filesytem
> is free to reallocate a block under active dma to another file and now
> the expected data-incoherency situation has turned into active
> data-corruption.
>
> Solution:
>
> Defer all filesystem operations (fallocate(), truncate()) on a dax mode
> file while any page/block in the file is under active dma. This solution
> assumes that dma is transient. Cases where dma operations are known to
> not be transient, like RDMA, have been explicitly disabled via
> commits like 5f1d43de5416 "IB/core: disable memory registration of
> filesystem-dax vmas".
>
> The dax_layout_busy_page() routine is called by filesystems with a lock
> held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
> The process of looking up a busy page invalidates all mappings
> to trigger any subsequent get_user_pages() to block on i_mmap_lock.
> The filesystem continues to call dax_layout_busy_page() until it finally
> returns no more active pages. This approach assumes that the page
> pinning is transient, if that assumption is violated the system would
> have likely hung from the uncompleted I/O.
>
> Cc: Jan Kara
> Cc: Jeff Moyer
> Cc: Dave Chinner
> Cc: Matthew Wilcox
> Cc: Alexander Viro
> Cc: "Darrick J. Wong"
> Cc: Ross Zwisler
> Cc: Dave Hansen
> Cc: Andrew Morton
> Reported-by: Christoph Hellwig
> Reviewed-by: Christoph Hellwig
> Signed-off-by: Dan Williams
> ---
> drivers/dax/super.c |2 +
> fs/dax.c| 92
> +++
> include/linux/dax.h | 25 ++
> mm/gup.c|5 +++
> 4 files changed, 123 insertions(+), 1 deletion(-)
...
> +/**
> + * dax_layout_busy_page - find first pinned page in @mapping
> + * @mapping: address space to scan for a page with ref count > 1
> + *
> + * DAX requires ZONE_DEVICE mapped pages. These pages are never
> + * 'onlined' to the page allocator so they are considered idle when
> + * page->count == 1. A filesystem uses this interface to determine if
> + * any page in the mapping is busy, i.e. for DMA, or other
> + * get_user_pages() usages.
> + *
> + * It is expected that the filesystem is holding locks to block the
> + * establishment of new mappings in this address_space. I.e. it expects
> + * to be able to run unmap_mapping_range() and subsequently not race
> + * mapping_mapped() becoming true. It expects that get_user_pages() pte
> + * walks are performed under rcu_read_lock().
> + */
> +struct page *dax_layout_busy_page(struct address_space *mapping)
> +{
> + pgoff_t indices[PAGEVEC_SIZE];
> + struct page *page = NULL;
> + struct pagevec pvec;
> + pgoff_t index, end;
> + unsigned i;
> +
> + /*
> + * In the 'limited' case get_user_pages() for dax is disabled.
> + */
> + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> + return NULL;
> +
> + if (!dax_mapping(mapping) || !mapping_mapped(mapping))
> + return NULL;
> +
> + pagevec_init(&pvec);
> + index = 0;
> + end = -1;
> + /*
> + * Flush dax_layout_lock() sections to ensure all possible page
> + * references have been taken, or otherwise arrange for faults
> + * to block on the filesystem lock that is taken for
> + * establishing new mappings.
> + */
> + unmap_mapping_range(mapping, 0, 0, 1);
> + synchronize_rcu();
So I still don't like the use of RCU for this. It just seems as an abuse to
use RCU like that. Furthermore it has a hefty latency cost for the truncate
path. A trivial test to truncate 100 times the last page of a 16k file that
is mmaped (only the first page):
DAX+your patches3.899s
non-DAX 0.015s
So you can see synchronize_rcu() increased time to run truncate(2) more
than 200 times (the process is indeed sitting in __wait_rcu_gp all the
time). IMHO that's just too costly.
> + while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
> + min(end - index, (pgoff_t)
