Re: [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)
On 23.10.19 09:26, David Hildenbrand wrote:
On 22.10.19 23:54, Dan Williams wrote:
Hi David,
Thanks for tackling this!
Thanks for having a look :)
[...]
I am probably a little bit too careful (but I don't want to break things).
In most places (besides KVM and vfio that are nuts), the
pfn_to_online_page() check could most probably be avoided by a
is_zone_device_page() check. However, I usually get suspicious when I see
a pfn_valid() check (especially after I learned that people mmap parts of
/dev/mem into user space, including memory without memmaps. Also, people
could memmap offline memory blocks this way :/). As long as this does not
hurt performance, I think we should rather do it the clean way.
I'm concerned about using is_zone_device_page() in places that are not
known to already have a reference to the page. Here's an audit of
current usages, and the ones I think need to cleaned up. The "unsafe"
ones do not appear to have any protections against the device page
being removed (get_dev_pagemap()). Yes, some of these were added by
me. The "unsafe? HMM" ones need HMM eyes because HMM leaks device
pages into anonymous memory paths and I'm not up to speed on how it
guarantees 'struct page' validity vs device shutdown without using
get_dev_pagemap().
smaps_pmd_entry(): unsafe
put_devmap_managed_page(): safe, page reference is held
is_device_private_page(): safe? gpu driver manages private page lifetime
is_pci_p2pdma_page(): safe, page reference is held
uncharge_page(): unsafe? HMM
add_to_kill(): safe, protected by get_dev_pagemap() and dax_lock_page()
soft_offline_page(): unsafe
remove_migration_pte(): unsafe? HMM
move_to_new_page(): unsafe? HMM
migrate_vma_pages() and helpers: unsafe? HMM
try_to_unmap_one(): unsafe? HMM
__put_page(): safe
release_pages(): safe
I'm hoping all the HMM ones can be converted to
is_device_private_page() directlly and have that routine grow a nice
comment about how it knows it can always safely de-reference its @page
argument.
For the rest I'd like to propose that we add a facility to determine
ZONE_DEVICE by pfn rather than page. The most straightforward why I
can think of would be to just add another bitmap to mem_section_usage
to indicate if a subsection is ZONE_DEVICE or not.
(it's a somewhat unrelated bigger discussion, but we can start discussing it in
this thread)
I dislike this for three reasons
a) It does not protect against any races, really, it does not improve things.
b) We do have the exact same problem with pfn_to_online_page(). As long as we
don't hold the memory hotplug lock, memory can get offlined and remove any
time. Racy.
c) We mix in ZONE specific stuff into the core. It should be "just another zone"
What I propose instead (already discussed in
https://lkml.org/lkml/2019/10/10/87)
1. Convert SECTION_IS_ONLINE to SECTION_IS_ACTIVE
2. Convert SECTION_IS_ACTIVE to a subsection bitmap
3. Introduce pfn_active() that checks against the subsection bitmap
4. Once the memmap was initialized / prepared, set the subsection active
(similar to SECTION_IS_ONLINE in the buddy right now)
5. Before the memmap gets invalidated, set the subsection inactive
(similar to SECTION_IS_ONLINE in the buddy right now)
5. pfn_to_online_page() = pfn_active() && zone != ZONE_DEVICE
6. pfn_to_device_page() = pfn_active() && zone == ZONE_DEVICE
Dan, I am suspecting that you want a pfn_to_zone() that will not touch
the memmap, because it could potentially (altmap) lie on slow memory, right?
A modification might make this possible (but I am not yet sure if we
want a less generic MM implementation just to fine tune slow memmap
access here)
1. Keep SECTION_IS_ONLINE as it is with the same semantics
2. Introduce a subsection bitmap to record active ("initialized memmap")
PFNs. E.g., also set it when setting sections online.
3. Introduce pfn_active() that checks against the subsection bitmap
4. Once the memmap was initialized / prepared, set the subsection active
(similar to SECTION_IS_ONLINE in the buddy right now)
5. Before the memmap gets invalidated, set the subsection inactive
(similar to SECTION_IS_ONLINE in the buddy right now)
5. pfn_to_online_page() = pfn_active() && section == SECTION_IS_ONLINE
(or keep it as is, depends on the RCU locking we eventually
implement)
6. pfn_to_device_page() = pfn_active() && section != SECTION_IS_ONLINE
7. use pfn_active() whenever we don't care about the zone.
Again, not really a friend of that, it hardcodes ZONE_DEVICE vs.
!ZONE_DEVICE. When we do a random "pfn_to_page()" (e.g., a pfn walker)
we really want to touch the memmap right away either way. So we can also
directly read the zone from it. I really do prefer right now a more
generic implementation.
--
Thanks,
David / dhildenb
Re: [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)
On 23.10.19 21:39, Dan Williams wrote: > On Wed, Oct 23, 2019 at 10:28 AM David Hildenbrand wrote: >> I dislike this for three reasons a) It does not protect against any races, really, it does not improve things. b) We do have the exact same problem with pfn_to_online_page(). As long as we don't hold the memory hotplug lock, memory can get offlined and remove any time. Racy. >>> >>> True, we need to solve that problem too. That seems to want something >>> lighter weight than the hotplug lock that can be held over pfn lookups >>> + use rather than requiring a page lookup in paths where it's not >>> clear that a page reference would prevent unplug. >>> c) We mix in ZONE specific stuff into the core. It should be "just another zone" >>> >>> Not sure I grok this when the RFC is sprinkling zone-specific >>> is_zone_device_page() throughout the core? >> >> Most users should not care about the zone. pfn_active() would be enough >> in most situations, especially most PFN walkers - "this memmap is valid >> and e.g., contains a valid zone ...". > > Oh, I see, you're saying convert most users to pfn_active() (and some > TBD rcu locking), but only pfn_to_online_page() users would need the > zone lookup? I can get on board with that. I guess my answer to that is simple: If we only care about "is this memmap safe to touch", use pfn_active() (well, with pfn_valid_within() similar as done in pfn_to_online_page() due to memory holes, but these are details - e.g., pfn_active() can check against pfn_valid_within() right away internally). (+locking TBD to make sure it remains active) However, if we want to special case in addition on zones (!ZONE_DEVICE (a.k.a., onlined via memory blocks, managed by the buddy), ZONE_DEVICE, whatever might come in the future, ...), also access the zone stored in the memmap. E.g., by using pfn_to_online_page(). > >> >>> What I propose instead (already discussed in https://lkml.org/lkml/2019/10/10/87) >>> >>> Sorry I missed this earlier... >>> 1. Convert SECTION_IS_ONLINE to SECTION_IS_ACTIVE 2. Convert SECTION_IS_ACTIVE to a subsection bitmap 3. Introduce pfn_active() that checks against the subsection bitmap 4. Once the memmap was initialized / prepared, set the subsection active (similar to SECTION_IS_ONLINE in the buddy right now) 5. Before the memmap gets invalidated, set the subsection inactive (similar to SECTION_IS_ONLINE in the buddy right now) 5. pfn_to_online_page() = pfn_active() && zone != ZONE_DEVICE 6. pfn_to_device_page() = pfn_active() && zone == ZONE_DEVICE >>> >>> This does not seem to reduce any complexity because it still requires >>> a pfn to zone lookup at the end of the process. >>> >>> I.e. converting pfn_to_online_page() to use a new pfn_active() >>> subsection map plus looking up the zone from pfn_to_page() is more >>> steps than just doing a direct pfn to zone lookup. What am I missing? >> >> That a real "pfn to zone" lookup without going via the struct page will >> require to have more than just a single bitmap. IMHO, keeping the >> information at a single place (memmap) is the clean thing to do (not >> replicating it somewhere else). Going via the memmap might not be as >> fast as a direct lookup, but do we actually care? We are already looking >> at "random PFNs we are not sure if there is a valid memmap". > > True, we only care about the validity of the check, and as you pointed > out moving the check to the pfn level does not solve the validity > race. It needs a lock. Let's call pfn_active() "a pfn that is active in the system and has an initialized memmap, which contains sane values" (valid memmap sounds like pfn_valid(), which is actually "there is a memmap which might contain garbage"). Yes we need some sort of lightweight locking as discussed. [...] However, I think we also have to clarify if we need the change in 3 at all. It comes from commit f5509cc18daa7f82bcc553be70df2117c8eedc16 Author: Kees Cook Date: Tue Jun 7 11:05:33 2016 -0700 mm: Hardened usercopy This is the start of porting PAX_USERCOPY into the mainline kernel. This is the first set of features, controlled by CONFIG_HARDENED_USERCOPY. The work is based on code by PaX Team and Brad Spengler, and an earlier port from Casey Schaufler. Additional non-slab page tests are from Rik van Riel. [...] - otherwise, object must not span page allocations (excepting Reserved and CMA ranges) Not sure if we really have to care about ZONE_DEVICE at this point. >>> >>> That check needs to be careful to ignore ZONE_DEVICE pages. There's >>> nothing wrong with a copy spanning ZONE_DEVICE and typical pages. >> >> Please note that the current check would *forbid* this (AFAIKs for a >> single heap object). As discussed in the
Re: [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)
On Wed, Oct 23, 2019 at 10:28 AM David Hildenbrand wrote:
>
> >> I dislike this for three reasons
> >>
> >> a) It does not protect against any races, really, it does not improve
> >> things.
> >> b) We do have the exact same problem with pfn_to_online_page(). As long as
> >> we
> >>don't hold the memory hotplug lock, memory can get offlined and remove
> >> any time. Racy.
> >
> > True, we need to solve that problem too. That seems to want something
> > lighter weight than the hotplug lock that can be held over pfn lookups
> > + use rather than requiring a page lookup in paths where it's not
> > clear that a page reference would prevent unplug.
> >
> >> c) We mix in ZONE specific stuff into the core. It should be "just another
> >> zone"
> >
> > Not sure I grok this when the RFC is sprinkling zone-specific
> > is_zone_device_page() throughout the core?
>
> Most users should not care about the zone. pfn_active() would be enough
> in most situations, especially most PFN walkers - "this memmap is valid
> and e.g., contains a valid zone ...".
Oh, I see, you're saying convert most users to pfn_active() (and some
TBD rcu locking), but only pfn_to_online_page() users would need the
zone lookup? I can get on board with that.
>
> >
> >>
> >> What I propose instead (already discussed in
> >> https://lkml.org/lkml/2019/10/10/87)
> >
> > Sorry I missed this earlier...
> >
> >>
> >> 1. Convert SECTION_IS_ONLINE to SECTION_IS_ACTIVE
> >> 2. Convert SECTION_IS_ACTIVE to a subsection bitmap
> >> 3. Introduce pfn_active() that checks against the subsection bitmap
> >> 4. Once the memmap was initialized / prepared, set the subsection active
> >>(similar to SECTION_IS_ONLINE in the buddy right now)
> >> 5. Before the memmap gets invalidated, set the subsection inactive
> >>(similar to SECTION_IS_ONLINE in the buddy right now)
> >> 5. pfn_to_online_page() = pfn_active() && zone != ZONE_DEVICE
> >> 6. pfn_to_device_page() = pfn_active() && zone == ZONE_DEVICE
> >
> > This does not seem to reduce any complexity because it still requires
> > a pfn to zone lookup at the end of the process.
> >
> > I.e. converting pfn_to_online_page() to use a new pfn_active()
> > subsection map plus looking up the zone from pfn_to_page() is more
> > steps than just doing a direct pfn to zone lookup. What am I missing?
>
> That a real "pfn to zone" lookup without going via the struct page will
> require to have more than just a single bitmap. IMHO, keeping the
> information at a single place (memmap) is the clean thing to do (not
> replicating it somewhere else). Going via the memmap might not be as
> fast as a direct lookup, but do we actually care? We are already looking
> at "random PFNs we are not sure if there is a valid memmap".
True, we only care about the validity of the check, and as you pointed
out moving the check to the pfn level does not solve the validity
race. It needs a lock.
>
> >>
> >> Especially, driver-reserved device memory will not get set active in
> >> the subsection bitmap.
> >>
> >> Now to the race. Taking the memory hotplug lock at random places is ugly.
> >> I do
> >> wonder if we can use RCU:
> >
> > Ah, yes, exactly what I was thinking above.
> >
> >>
> >> The user of pfn_active()/pfn_to_online_page()/pfn_to_device_page():
> >>
> >> /* the memmap is guaranteed to remain active under RCU */
> >> rcu_read_lock();
> >> if (pfn_active(random_pfn)) {
> >> page = pfn_to_page(random_pfn);
> >> ... use the page, stays valid
> >> }
> >> rcu_unread_lock();
> >>
> >> Memory offlining/memremap code:
> >>
> >> set_subsections_inactive(pfn, nr_pages); /* clears the bit
> >> atomically */
> >> synchronize_rcu();
> >> /* all users saw the bitmap update, we can invalide the memmap */
> >> remove_pfn_range_from_zone(zone, pfn, nr_pages);
> >
> > Looks good to me.
> >
> >>
> >>>
>
> I only gave it a quick test with DIMMs on x86-64, but didn't test the
> ZONE_DEVICE part at all (any tips for a nice QEMU setup?). Compile-tested
> on x86-64 and PPC.
> >>>
> >>> I'll give it a spin, but I don't think the kernel wants to grow more
> >>> is_zone_device_page() users.
> >>
> >> Let's recap. In this RFC, I introduce a total of 4 (!) users only.
> >> The other parts can rely on pfn_to_online_page() only.
> >>
> >> 1. "staging: kpc2000: Prepare transfer_complete_cb() for PG_reserved
> >> changes"
> >> - Basically never used with ZONE_DEVICE.
> >> - We hold a reference!
> >> - All it protects is a SetPageDirty(page);
> >>
> >> 2. "staging/gasket: Prepare gasket_release_page() for PG_reserved changes"
> >> - Same as 1.
> >>
> >> 3. "mm/usercopy.c: Prepare check_page_span() for PG_reserved changes"
> >> - We come via virt_to_head_page() / virt_to_head_page(), not sure about
> >> references (I assume this should be fine as we don't come via random
> >> PFNs)
> >> - We check that we don't mix Reserved
Re: [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)
>> I dislike this for three reasons
>>
>> a) It does not protect against any races, really, it does not improve things.
>> b) We do have the exact same problem with pfn_to_online_page(). As long as we
>>don't hold the memory hotplug lock, memory can get offlined and remove
>> any time. Racy.
>
> True, we need to solve that problem too. That seems to want something
> lighter weight than the hotplug lock that can be held over pfn lookups
> + use rather than requiring a page lookup in paths where it's not
> clear that a page reference would prevent unplug.
>
>> c) We mix in ZONE specific stuff into the core. It should be "just another
>> zone"
>
> Not sure I grok this when the RFC is sprinkling zone-specific
> is_zone_device_page() throughout the core?
Most users should not care about the zone. pfn_active() would be enough
in most situations, especially most PFN walkers - "this memmap is valid
and e.g., contains a valid zone ...".
>
>>
>> What I propose instead (already discussed in
>> https://lkml.org/lkml/2019/10/10/87)
>
> Sorry I missed this earlier...
>
>>
>> 1. Convert SECTION_IS_ONLINE to SECTION_IS_ACTIVE
>> 2. Convert SECTION_IS_ACTIVE to a subsection bitmap
>> 3. Introduce pfn_active() that checks against the subsection bitmap
>> 4. Once the memmap was initialized / prepared, set the subsection active
>>(similar to SECTION_IS_ONLINE in the buddy right now)
>> 5. Before the memmap gets invalidated, set the subsection inactive
>>(similar to SECTION_IS_ONLINE in the buddy right now)
>> 5. pfn_to_online_page() = pfn_active() && zone != ZONE_DEVICE
>> 6. pfn_to_device_page() = pfn_active() && zone == ZONE_DEVICE
>
> This does not seem to reduce any complexity because it still requires
> a pfn to zone lookup at the end of the process.
>
> I.e. converting pfn_to_online_page() to use a new pfn_active()
> subsection map plus looking up the zone from pfn_to_page() is more
> steps than just doing a direct pfn to zone lookup. What am I missing?
That a real "pfn to zone" lookup without going via the struct page will
require to have more than just a single bitmap. IMHO, keeping the
information at a single place (memmap) is the clean thing to do (not
replicating it somewhere else). Going via the memmap might not be as
fast as a direct lookup, but do we actually care? We are already looking
at "random PFNs we are not sure if there is a valid memmap".
>>
>> Especially, driver-reserved device memory will not get set active in
>> the subsection bitmap.
>>
>> Now to the race. Taking the memory hotplug lock at random places is ugly. I
>> do
>> wonder if we can use RCU:
>
> Ah, yes, exactly what I was thinking above.
>
>>
>> The user of pfn_active()/pfn_to_online_page()/pfn_to_device_page():
>>
>> /* the memmap is guaranteed to remain active under RCU */
>> rcu_read_lock();
>> if (pfn_active(random_pfn)) {
>> page = pfn_to_page(random_pfn);
>> ... use the page, stays valid
>> }
>> rcu_unread_lock();
>>
>> Memory offlining/memremap code:
>>
>> set_subsections_inactive(pfn, nr_pages); /* clears the bit
>> atomically */
>> synchronize_rcu();
>> /* all users saw the bitmap update, we can invalide the memmap */
>> remove_pfn_range_from_zone(zone, pfn, nr_pages);
>
> Looks good to me.
>
>>
>>>
I only gave it a quick test with DIMMs on x86-64, but didn't test the
ZONE_DEVICE part at all (any tips for a nice QEMU setup?). Compile-tested
on x86-64 and PPC.
>>>
>>> I'll give it a spin, but I don't think the kernel wants to grow more
>>> is_zone_device_page() users.
>>
>> Let's recap. In this RFC, I introduce a total of 4 (!) users only.
>> The other parts can rely on pfn_to_online_page() only.
>>
>> 1. "staging: kpc2000: Prepare transfer_complete_cb() for PG_reserved changes"
>> - Basically never used with ZONE_DEVICE.
>> - We hold a reference!
>> - All it protects is a SetPageDirty(page);
>>
>> 2. "staging/gasket: Prepare gasket_release_page() for PG_reserved changes"
>> - Same as 1.
>>
>> 3. "mm/usercopy.c: Prepare check_page_span() for PG_reserved changes"
>> - We come via virt_to_head_page() / virt_to_head_page(), not sure about
>> references (I assume this should be fine as we don't come via random
>> PFNs)
>> - We check that we don't mix Reserved (including device memory) and CMA
>> pages when crossing compound pages.
>>
>> I think we can drop 1. and 2., resulting in a total of 2 new users in
>> the same context. I think that is totally tolerable to finally clean
>> this up.
>
> ...but more is_zone_device_page() doesn't "finally clean this up".
> Like we discussed above it's the missing locking that's the real
> cleanup, the pfn_to_online_page() internals are secondary.
It's a different cleanup IMHO. We can't do everything in one shot. But
maybe I can drop the is_zone_device_page() parts from this patch and
completely rely on pfn_to_online_page(). Yes, that
Re: [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)
On Wed, Oct 23, 2019 at 12:26 AM David Hildenbrand wrote:
>
> On 22.10.19 23:54, Dan Williams wrote:
> > Hi David,
> >
> > Thanks for tackling this!
>
> Thanks for having a look :)
>
> [...]
>
>
> >> I am probably a little bit too careful (but I don't want to break things).
> >> In most places (besides KVM and vfio that are nuts), the
> >> pfn_to_online_page() check could most probably be avoided by a
> >> is_zone_device_page() check. However, I usually get suspicious when I see
> >> a pfn_valid() check (especially after I learned that people mmap parts of
> >> /dev/mem into user space, including memory without memmaps. Also, people
> >> could memmap offline memory blocks this way :/). As long as this does not
> >> hurt performance, I think we should rather do it the clean way.
> >
> > I'm concerned about using is_zone_device_page() in places that are not
> > known to already have a reference to the page. Here's an audit of
> > current usages, and the ones I think need to cleaned up. The "unsafe"
> > ones do not appear to have any protections against the device page
> > being removed (get_dev_pagemap()). Yes, some of these were added by
> > me. The "unsafe? HMM" ones need HMM eyes because HMM leaks device
> > pages into anonymous memory paths and I'm not up to speed on how it
> > guarantees 'struct page' validity vs device shutdown without using
> > get_dev_pagemap().
> >
> > smaps_pmd_entry(): unsafe
> >
> > put_devmap_managed_page(): safe, page reference is held
> >
> > is_device_private_page(): safe? gpu driver manages private page lifetime
> >
> > is_pci_p2pdma_page(): safe, page reference is held
> >
> > uncharge_page(): unsafe? HMM
> >
> > add_to_kill(): safe, protected by get_dev_pagemap() and dax_lock_page()
> >
> > soft_offline_page(): unsafe
> >
> > remove_migration_pte(): unsafe? HMM
> >
> > move_to_new_page(): unsafe? HMM
> >
> > migrate_vma_pages() and helpers: unsafe? HMM
> >
> > try_to_unmap_one(): unsafe? HMM
> >
> > __put_page(): safe
> >
> > release_pages(): safe
> >
> > I'm hoping all the HMM ones can be converted to
> > is_device_private_page() directlly and have that routine grow a nice
> > comment about how it knows it can always safely de-reference its @page
> > argument.
> >
> > For the rest I'd like to propose that we add a facility to determine
> > ZONE_DEVICE by pfn rather than page. The most straightforward why I
> > can think of would be to just add another bitmap to mem_section_usage
> > to indicate if a subsection is ZONE_DEVICE or not.
>
> (it's a somewhat unrelated bigger discussion, but we can start discussing it
> in this thread)
>
> I dislike this for three reasons
>
> a) It does not protect against any races, really, it does not improve things.
> b) We do have the exact same problem with pfn_to_online_page(). As long as we
>don't hold the memory hotplug lock, memory can get offlined and remove any
> time. Racy.
True, we need to solve that problem too. That seems to want something
lighter weight than the hotplug lock that can be held over pfn lookups
+ use rather than requiring a page lookup in paths where it's not
clear that a page reference would prevent unplug.
> c) We mix in ZONE specific stuff into the core. It should be "just another
> zone"
Not sure I grok this when the RFC is sprinkling zone-specific
is_zone_device_page() throughout the core?
>
> What I propose instead (already discussed in
> https://lkml.org/lkml/2019/10/10/87)
Sorry I missed this earlier...
>
> 1. Convert SECTION_IS_ONLINE to SECTION_IS_ACTIVE
> 2. Convert SECTION_IS_ACTIVE to a subsection bitmap
> 3. Introduce pfn_active() that checks against the subsection bitmap
> 4. Once the memmap was initialized / prepared, set the subsection active
>(similar to SECTION_IS_ONLINE in the buddy right now)
> 5. Before the memmap gets invalidated, set the subsection inactive
>(similar to SECTION_IS_ONLINE in the buddy right now)
> 5. pfn_to_online_page() = pfn_active() && zone != ZONE_DEVICE
> 6. pfn_to_device_page() = pfn_active() && zone == ZONE_DEVICE
This does not seem to reduce any complexity because it still requires
a pfn to zone lookup at the end of the process.
I.e. converting pfn_to_online_page() to use a new pfn_active()
subsection map plus looking up the zone from pfn_to_page() is more
steps than just doing a direct pfn to zone lookup. What am I missing?
>
> Especially, driver-reserved device memory will not get set active in
> the subsection bitmap.
>
> Now to the race. Taking the memory hotplug lock at random places is ugly. I do
> wonder if we can use RCU:
Ah, yes, exactly what I was thinking above.
>
> The user of pfn_active()/pfn_to_online_page()/pfn_to_device_page():
>
> /* the memmap is guaranteed to remain active under RCU */
> rcu_read_lock();
> if (pfn_active(random_pfn)) {
> page = pfn_to_page(random_pfn);
> ... use the page, stays valid
> }
> rcu_unread_lock();
>
> Memory
Re: [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)
On 22.10.19 23:54, Dan Williams wrote:
> Hi David,
>
> Thanks for tackling this!
Thanks for having a look :)
[...]
>> I am probably a little bit too careful (but I don't want to break things).
>> In most places (besides KVM and vfio that are nuts), the
>> pfn_to_online_page() check could most probably be avoided by a
>> is_zone_device_page() check. However, I usually get suspicious when I see
>> a pfn_valid() check (especially after I learned that people mmap parts of
>> /dev/mem into user space, including memory without memmaps. Also, people
>> could memmap offline memory blocks this way :/). As long as this does not
>> hurt performance, I think we should rather do it the clean way.
>
> I'm concerned about using is_zone_device_page() in places that are not
> known to already have a reference to the page. Here's an audit of
> current usages, and the ones I think need to cleaned up. The "unsafe"
> ones do not appear to have any protections against the device page
> being removed (get_dev_pagemap()). Yes, some of these were added by
> me. The "unsafe? HMM" ones need HMM eyes because HMM leaks device
> pages into anonymous memory paths and I'm not up to speed on how it
> guarantees 'struct page' validity vs device shutdown without using
> get_dev_pagemap().
>
> smaps_pmd_entry(): unsafe
>
> put_devmap_managed_page(): safe, page reference is held
>
> is_device_private_page(): safe? gpu driver manages private page lifetime
>
> is_pci_p2pdma_page(): safe, page reference is held
>
> uncharge_page(): unsafe? HMM
>
> add_to_kill(): safe, protected by get_dev_pagemap() and dax_lock_page()
>
> soft_offline_page(): unsafe
>
> remove_migration_pte(): unsafe? HMM
>
> move_to_new_page(): unsafe? HMM
>
> migrate_vma_pages() and helpers: unsafe? HMM
>
> try_to_unmap_one(): unsafe? HMM
>
> __put_page(): safe
>
> release_pages(): safe
>
> I'm hoping all the HMM ones can be converted to
> is_device_private_page() directlly and have that routine grow a nice
> comment about how it knows it can always safely de-reference its @page
> argument.
>
> For the rest I'd like to propose that we add a facility to determine
> ZONE_DEVICE by pfn rather than page. The most straightforward why I
> can think of would be to just add another bitmap to mem_section_usage
> to indicate if a subsection is ZONE_DEVICE or not.
(it's a somewhat unrelated bigger discussion, but we can start discussing it in
this thread)
I dislike this for three reasons
a) It does not protect against any races, really, it does not improve things.
b) We do have the exact same problem with pfn_to_online_page(). As long as we
don't hold the memory hotplug lock, memory can get offlined and remove any
time. Racy.
c) We mix in ZONE specific stuff into the core. It should be "just another zone"
What I propose instead (already discussed in
https://lkml.org/lkml/2019/10/10/87)
1. Convert SECTION_IS_ONLINE to SECTION_IS_ACTIVE
2. Convert SECTION_IS_ACTIVE to a subsection bitmap
3. Introduce pfn_active() that checks against the subsection bitmap
4. Once the memmap was initialized / prepared, set the subsection active
(similar to SECTION_IS_ONLINE in the buddy right now)
5. Before the memmap gets invalidated, set the subsection inactive
(similar to SECTION_IS_ONLINE in the buddy right now)
5. pfn_to_online_page() = pfn_active() && zone != ZONE_DEVICE
6. pfn_to_device_page() = pfn_active() && zone == ZONE_DEVICE
Especially, driver-reserved device memory will not get set active in
the subsection bitmap.
Now to the race. Taking the memory hotplug lock at random places is ugly. I do
wonder if we can use RCU:
The user of pfn_active()/pfn_to_online_page()/pfn_to_device_page():
/* the memmap is guaranteed to remain active under RCU */
rcu_read_lock();
if (pfn_active(random_pfn)) {
page = pfn_to_page(random_pfn);
... use the page, stays valid
}
rcu_unread_lock();
Memory offlining/memremap code:
set_subsections_inactive(pfn, nr_pages); /* clears the bit atomically */
synchronize_rcu();
/* all users saw the bitmap update, we can invalide the memmap */
remove_pfn_range_from_zone(zone, pfn, nr_pages);
>
>>
>> I only gave it a quick test with DIMMs on x86-64, but didn't test the
>> ZONE_DEVICE part at all (any tips for a nice QEMU setup?). Compile-tested
>> on x86-64 and PPC.
>
> I'll give it a spin, but I don't think the kernel wants to grow more
> is_zone_device_page() users.
Let's recap. In this RFC, I introduce a total of 4 (!) users only.
The other parts can rely on pfn_to_online_page() only.
1. "staging: kpc2000: Prepare transfer_complete_cb() for PG_reserved changes"
- Basically never used with ZONE_DEVICE.
- We hold a reference!
- All it protects is a SetPageDirty(page);
2. "staging/gasket: Prepare gasket_release_page() for PG_reserved changes"
- Same as 1.
3. "mm/usercopy.c: Prepare check_page_span() for PG_reserved changes"
- We
Re: [PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)
Hi David, Thanks for tackling this! On Tue, Oct 22, 2019 at 10:13 AM David Hildenbrand wrote: > > This series is based on [2], which should pop up in linux/next soon: > https://lkml.org/lkml/2019/10/21/1034 > > This is the result of a recent discussion with Michal ([1], [2]). Right > now we set all pages PG_reserved when initializing hotplugged memmaps. This > includes ZONE_DEVICE memory. In case of system memory, PG_reserved is > cleared again when onlining the memory, in case of ZONE_DEVICE memory > never. In ancient times, we needed PG_reserved, because there was no way > to tell whether the memmap was already properly initialized. We now have > SECTION_IS_ONLINE for that in the case of !ZONE_DEVICE memory. ZONE_DEVICE > memory is already initialized deferred, and there shouldn't be a visible > change in that regard. > > I remember that some time ago, we already talked about stopping to set > ZONE_DEVICE pages PG_reserved on the list, but I never saw any patches. > Also, I forgot who was part of the discussion :) You got me, Alex, and KVM folks on the Cc, so I'd say that was it. > One of the biggest fear were side effects. I went ahead and audited all > users of PageReserved(). The ones that don't need any care (patches) > can be found below. I will double check and hope I am not missing something > important. > > I am probably a little bit too careful (but I don't want to break things). > In most places (besides KVM and vfio that are nuts), the > pfn_to_online_page() check could most probably be avoided by a > is_zone_device_page() check. However, I usually get suspicious when I see > a pfn_valid() check (especially after I learned that people mmap parts of > /dev/mem into user space, including memory without memmaps. Also, people > could memmap offline memory blocks this way :/). As long as this does not > hurt performance, I think we should rather do it the clean way. I'm concerned about using is_zone_device_page() in places that are not known to already have a reference to the page. Here's an audit of current usages, and the ones I think need to cleaned up. The "unsafe" ones do not appear to have any protections against the device page being removed (get_dev_pagemap()). Yes, some of these were added by me. The "unsafe? HMM" ones need HMM eyes because HMM leaks device pages into anonymous memory paths and I'm not up to speed on how it guarantees 'struct page' validity vs device shutdown without using get_dev_pagemap(). smaps_pmd_entry(): unsafe put_devmap_managed_page(): safe, page reference is held is_device_private_page(): safe? gpu driver manages private page lifetime is_pci_p2pdma_page(): safe, page reference is held uncharge_page(): unsafe? HMM add_to_kill(): safe, protected by get_dev_pagemap() and dax_lock_page() soft_offline_page(): unsafe remove_migration_pte(): unsafe? HMM move_to_new_page(): unsafe? HMM migrate_vma_pages() and helpers: unsafe? HMM try_to_unmap_one(): unsafe? HMM __put_page(): safe release_pages(): safe I'm hoping all the HMM ones can be converted to is_device_private_page() directlly and have that routine grow a nice comment about how it knows it can always safely de-reference its @page argument. For the rest I'd like to propose that we add a facility to determine ZONE_DEVICE by pfn rather than page. The most straightforward why I can think of would be to just add another bitmap to mem_section_usage to indicate if a subsection is ZONE_DEVICE or not. > > I only gave it a quick test with DIMMs on x86-64, but didn't test the > ZONE_DEVICE part at all (any tips for a nice QEMU setup?). Compile-tested > on x86-64 and PPC. I'll give it a spin, but I don't think the kernel wants to grow more is_zone_device_page() users.
[PATCH RFC v1 00/12] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE)
This series is based on [2], which should pop up in linux/next soon: https://lkml.org/lkml/2019/10/21/1034 This is the result of a recent discussion with Michal ([1], [2]). Right now we set all pages PG_reserved when initializing hotplugged memmaps. This includes ZONE_DEVICE memory. In case of system memory, PG_reserved is cleared again when onlining the memory, in case of ZONE_DEVICE memory never. In ancient times, we needed PG_reserved, because there was no way to tell whether the memmap was already properly initialized. We now have SECTION_IS_ONLINE for that in the case of !ZONE_DEVICE memory. ZONE_DEVICE memory is already initialized deferred, and there shouldn't be a visible change in that regard. I remember that some time ago, we already talked about stopping to set ZONE_DEVICE pages PG_reserved on the list, but I never saw any patches. Also, I forgot who was part of the discussion :) One of the biggest fear were side effects. I went ahead and audited all users of PageReserved(). The ones that don't need any care (patches) can be found below. I will double check and hope I am not missing something important. I am probably a little bit too careful (but I don't want to break things). In most places (besides KVM and vfio that are nuts), the pfn_to_online_page() check could most probably be avoided by a is_zone_device_page() check. However, I usually get suspicious when I see a pfn_valid() check (especially after I learned that people mmap parts of /dev/mem into user space, including memory without memmaps. Also, people could memmap offline memory blocks this way :/). As long as this does not hurt performance, I think we should rather do it the clean way. I only gave it a quick test with DIMMs on x86-64, but didn't test the ZONE_DEVICE part at all (any tips for a nice QEMU setup?). Compile-tested on x86-64 and PPC. Other users of PageReserved() that should be fine: - mm/page_owner.c:pagetypeinfo_showmixedcount_print() -> Never called for ZONE_DEVICE, (+ pfn_to_online_page(pfn)) - mm/page_owner.c:init_pages_in_zone() -> Never called for ZONE_DEVICE (!populated_zone(zone)) - mm/page_ext.c:free_page_ext() -> Only a BUG_ON(PageReserved(page)), not relevant - mm/page_ext.c:has_unmovable_pages() -> Not releveant for ZONE_DEVICE - mm/page_ext.c:pfn_range_valid_contig() -> pfn_to_online_page() already guards us - mm/mempolicy.c:queue_pages_pte_range() -> vm_normal_page() checks against pte_devmap() - mm/memory-failure.c:hwpoison_user_mappings() -> Not reached via memory_failure() due to pfn_to_online_page() -> Also not reached indirectly via memory_failure_hugetlb() - mm/hugetlb.c:gather_bootmem_prealloc() -> Only a WARN_ON(PageReserved(page)), not relevant - kernel/power/snapshot.c:saveable_highmem_page() -> pfn_to_online_page() already guards us - kernel/power/snapshot.c:saveable_page() -> pfn_to_online_page() already guards us - fs/proc/task_mmu.c:can_gather_numa_stats() -> vm_normal_page() checks against pte_devmap() - fs/proc/task_mmu.c:can_gather_numa_stats_pmd -> vm_normal_page_pmd() checks against pte_devmap() - fs/proc/page.c:stable_page_flags() -> The reserved bit is simply copied, irrelevant - drivers/firmware/memmap.c:release_firmware_map_entry() -> really only a check to detect bootmem. Not relevant for ZONE_DEVICE - arch/ia64/kernel/mca_drv.c - arch/mips/mm/init.c - arch/mips/mm/ioremap.c - arch/nios2/mm/ioremap.c - arch/parisc/mm/ioremap.c - arch/sparc/mm/tlb.c - arch/xtensa/mm/cache.c -> No ZONE_DEVICE support - arch/powerpc/mm/init_64.c:vmemmap_free() -> Special-cases memmap on altmap -> Only a check for bootmem - arch/x86/kernel/alternative.c:__text_poke() -> Only a WARN_ON(!PageReserved(pages[0])) to verify it is bootmem - arch/x86/mm/init_64.c -> Only a check for bootmem [1] https://lkml.org/lkml/2019/10/21/736 [2] https://lkml.org/lkml/2019/10/21/1034 Cc: Michal Hocko Cc: Dan Williams Cc: kvm-...@vger.kernel.org Cc: linuxppc-dev@lists.ozlabs.org Cc: k...@vger.kernel.org Cc: linux-hyp...@vger.kernel.org Cc: de...@driverdev.osuosl.org Cc: xen-de...@lists.xenproject.org Cc: x...@kernel.org Cc: Alexander Duyck David Hildenbrand (12): mm/memory_hotplug: Don't allow to online/offline memory blocks with holes mm/usercopy.c: Prepare check_page_span() for PG_reserved changes KVM: x86/mmu: Prepare kvm_is_mmio_pfn() for PG_reserved changes KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes vfio/type1: Prepare is_invalid_reserved_pfn() for PG_reserved changes staging/gasket: Prepare gasket_release_page() for PG_reserved changes staging: kpc2000: Prepare transfer_complete_cb() for PG_reserved changes powerpc/book3s: Prepare kvmppc_book3s_instantiate_page() for PG_reserved changes powerpc/64s: Prepare hash_page_do_lazy_icache() for PG_reserved changes powerpc/mm: Prepare maybe_pte_to_page() for PG_reserved changes x86/mm: Prepare __ioremap_check_ram() for PG_reserved changes mm/memory_hotplug:
