Re: [PATCH 3/6] dax: Avoid page invalidation races and unnecessary radix tree traversals

[Jan Kara]

On Tue 29-11-16 15:31:38, Ross Zwisler wrote:
> On Thu, Nov 24, 2016 at 10:46:33AM +0100, Jan Kara wrote:
> > Currently each filesystem (possibly through generic_file_direct_write()
> > or iomap_dax_rw()) takes care of invalidating page tables and evicting
> 
> Just some nits about the commit message: the DAX I/O path function is now
> called dax_iomap_rw(), and no filesystems still use
> generic_file_direct_write() for DAX so you can probably remove it from the
> changelog - up to you.

Yeah, good spotting. Many things have changed through rebases. I've fixed
up the changelog to reflect current state.

> Aside from that:
> Reviewed-by: Ross Zwisler 

Thanks.

Honza

> 
> > hole pages from the radix tree when write(2) to the file happens. This
> > invalidation is only necessary when there is some block allocation
> > resulting from write(2). Furthermore in current place the invalidation
> > is racy wrt page fault instantiating a hole page just after we have
> > invalidated it.
> > 
> > So perform the page invalidation inside dax_do_io() where we can do it
> > only when really necessary and after blocks have been allocated so
> > nobody will be instantiating new hole pages anymore.
> > 
> > Reviewed-by: Christoph Hellwig 
> > Signed-off-by: Jan Kara 
> > ---
> >  fs/dax.c | 28 +++-
> >  1 file changed, 11 insertions(+), 17 deletions(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 4534f0e232e9..ddf77ef2ca18 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -984,6 +984,17 @@ dax_iomap_actor(struct inode *inode, loff_t pos, 
> > loff_t length, void *data,
> > if (WARN_ON_ONCE(iomap->type != IOMAP_MAPPED))
> > return -EIO;
> >  
> > +   /*
> > +* Write can allocate block for an area which has a hole page mapped
> > +* into page tables. We have to tear down these mappings so that data
> > +* written by write(2) is visible in mmap.
> > +*/
> > +   if ((iomap->flags & IOMAP_F_NEW) && inode->i_mapping->nrpages) {
> > +   invalidate_inode_pages2_range(inode->i_mapping,
> > + pos >> PAGE_SHIFT,
> > + (end - 1) >> PAGE_SHIFT);
> > +   }
> > +
> > while (pos < end) {
> > unsigned offset = pos & (PAGE_SIZE - 1);
> > struct blk_dax_ctl dax = { 0 };
> > @@ -1042,23 +1053,6 @@ dax_iomap_rw(struct kiocb *iocb, struct iov_iter 
> > *iter,
> > if (iov_iter_rw(iter) == WRITE)
> > flags |= IOMAP_WRITE;
> >  
> > -   /*
> > -* Yes, even DAX files can have page cache attached to them:  A zeroed
> > -* page is inserted into the pagecache when we have to serve a write
> > -* fault on a hole.  It should never be dirtied and can simply be
> > -* dropped from the pagecache once we get real data for the page.
> > -*
> > -* XXX: This is racy against mmap, and there's nothing we can do about
> > -* it. We'll eventually need to shift this down even further so that
> > -* we can check if we allocated blocks over a hole first.
> > -*/
> > -   if (mapping->nrpages) {
> > -   ret = invalidate_inode_pages2_range(mapping,
> > -   pos >> PAGE_SHIFT,
> > -   (pos + iov_iter_count(iter) - 1) >> PAGE_SHIFT);
> > -   WARN_ON_ONCE(ret);
> > -   }
> > -
> > while (iov_iter_count(iter)) {
> > ret = iomap_apply(inode, pos, iov_iter_count(iter), flags, ops,
> > iter, dax_iomap_actor);
> > -- 
> > 2.6.6
> > 
-- 
Jan Kara 
SUSE Labs, CR
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Re: [PATCH 3/6] dax: Avoid page invalidation races and unnecessary radix tree traversals

[Ross Zwisler]

On Thu, Nov 24, 2016 at 10:46:33AM +0100, Jan Kara wrote:
> Currently each filesystem (possibly through generic_file_direct_write()
> or iomap_dax_rw()) takes care of invalidating page tables and evicting

Just some nits about the commit message: the DAX I/O path function is now
called dax_iomap_rw(), and no filesystems still use
generic_file_direct_write() for DAX so you can probably remove it from the
changelog - up to you.

Aside from that:
Reviewed-by: Ross Zwisler 

> hole pages from the radix tree when write(2) to the file happens. This
> invalidation is only necessary when there is some block allocation
> resulting from write(2). Furthermore in current place the invalidation
> is racy wrt page fault instantiating a hole page just after we have
> invalidated it.
> 
> So perform the page invalidation inside dax_do_io() where we can do it
> only when really necessary and after blocks have been allocated so
> nobody will be instantiating new hole pages anymore.
> 
> Reviewed-by: Christoph Hellwig 
> Signed-off-by: Jan Kara 
> ---
>  fs/dax.c | 28 +++-
>  1 file changed, 11 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 4534f0e232e9..ddf77ef2ca18 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -984,6 +984,17 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t 
> length, void *data,
>   if (WARN_ON_ONCE(iomap->type != IOMAP_MAPPED))
>   return -EIO;
>  
> + /*
> +  * Write can allocate block for an area which has a hole page mapped
> +  * into page tables. We have to tear down these mappings so that data
> +  * written by write(2) is visible in mmap.
> +  */
> + if ((iomap->flags & IOMAP_F_NEW) && inode->i_mapping->nrpages) {
> + invalidate_inode_pages2_range(inode->i_mapping,
> +   pos >> PAGE_SHIFT,
> +   (end - 1) >> PAGE_SHIFT);
> + }
> +
>   while (pos < end) {
>   unsigned offset = pos & (PAGE_SIZE - 1);
>   struct blk_dax_ctl dax = { 0 };
> @@ -1042,23 +1053,6 @@ dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
>   if (iov_iter_rw(iter) == WRITE)
>   flags |= IOMAP_WRITE;
>  
> - /*
> -  * Yes, even DAX files can have page cache attached to them:  A zeroed
> -  * page is inserted into the pagecache when we have to serve a write
> -  * fault on a hole.  It should never be dirtied and can simply be
> -  * dropped from the pagecache once we get real data for the page.
> -  *
> -  * XXX: This is racy against mmap, and there's nothing we can do about
> -  * it. We'll eventually need to shift this down even further so that
> -  * we can check if we allocated blocks over a hole first.
> -  */
> - if (mapping->nrpages) {
> - ret = invalidate_inode_pages2_range(mapping,
> - pos >> PAGE_SHIFT,
> - (pos + iov_iter_count(iter) - 1) >> PAGE_SHIFT);
> - WARN_ON_ONCE(ret);
> - }
> -
>   while (iov_iter_count(iter)) {
>   ret = iomap_apply(inode, pos, iov_iter_count(iter), flags, ops,
>   iter, dax_iomap_actor);
> -- 
> 2.6.6
> 
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