[add jan kara to cc]
On Mon, Sep 04, 2017 at 11:43:53AM +1000, Dave Chinner wrote:
> On Sun, Sep 03, 2017 at 12:43:06AM -0700, Christoph Hellwig wrote:
> > On Sun, Sep 03, 2017 at 09:22:17AM +0500, Михаил Гаврилов wrote:
> > > [281502.961248] ------------[ cut here ]------------
> > > [281502.961257] kernel BUG at fs/xfs/xfs_aops.c:853!
> >
> > This is:
> >
> > bh = head = page_buffers(page);
> >
> > Which looks odd and like some sort of VM/writeback change might
> > have triggered that we get a page without buffers, despite always
> > creating buffers in iomap_begin/end and page_mkwrite.
>
> Pretty sure this can still happen when buffer_heads_over_limit comes
> true. In that case, shrink_active_list() will attempt to strip
> the bufferheads off the page even if it's a dirty page. i.e. this
> code:
>
> if (unlikely(buffer_heads_over_limit)) {
> if (page_has_private(page) && trylock_page(page)) {
> if (page_has_private(page))
> try_to_release_page(page, 0);
> unlock_page(page);
> }
> }
>
>
> There was some discussion about this a while back, the consensus was
> that it is a mm bug, but nobody wanted to add a PageDirty check
> to try_to_release_page() and so nothing ended up being done about
> it in the mm/ subsystem. Instead, filesystems needed to avoid it
> if it was a problem for them. Indeed, we fixed it in the filesystem
> in 4.8:
>
> 99579ccec4e2 xfs: skip dirty pages in ->releasepage()
>
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 3ba0809e0be8..6135787500fc 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -1040,6 +1040,20 @@ xfs_vm_releasepage(
>
> trace_xfs_releasepage(page->mapping->host, page, 0, 0);
>
> + /*
> + * mm accommodates an old ext3 case where clean pages might not have had
> + * the dirty bit cleared. Thus, it can send actual dirty pages to
> + * ->releasepage() via shrink_active_list(). Conversely,
> + * block_invalidatepage() can send pages that are still marked dirty
> + * but otherwise have invalidated buffers.
> + *
> + * We've historically freed buffers on the latter. Instead, quietly
> + * filter out all dirty pages to avoid spurious buffer state warnings.
> + * This can likely be removed once shrink_active_list() is fixed.
> + */
> + if (PageDirty(page))
> + return 0;
> +
> xfs_count_page_state(page, &delalloc, &unwritten);
>
> But looking at the current code, the comment is still mostly there
> but the PageDirty() check isn't.
>
> <sigh>
>
> In 4.10, this was done:
>
> commit 0a417b8dc1f10b03e8f558b8a831f07ec4c23795
> Author: Jan Kara <jack@xxxxxxx>
> Date: Wed Jan 11 10:20:04 2017 -0800
>
> xfs: Timely free truncated dirty pages
>
> Commit 99579ccec4e2 "xfs: skip dirty pages in ->releasepage()" started
> to skip dirty pages in xfs_vm_releasepage() which also has the effect
> that if a dirty page is truncated, it does not get freed by
> block_invalidatepage() and is lingering in LRU list waiting for reclaim.
> So a simple loop like:
>
> while true; do
> dd if=/dev/zero of=file bs=1M count=100
> rm file
> done
>
> will keep using more and more memory until we hit low watermarks and
> start pagecache reclaim which will eventually reclaim also the truncate
> pages. Keeping these truncated (and thus never usable) pages in memory
> is just a waste of memory, is unnecessarily stressing page cache
> reclaim, and reportedly also leads to anonymous mmap(2) returning ENOMEM
> prematurely.
>
> So instead of just skipping dirty pages in xfs_vm_releasepage(), return
> to old behavior of skipping them only if they have delalloc or unwritten
> buffers and fix the spurious warnings by warning only if the page is
> clean.
>
> CC: stable@xxxxxxxxxxxxxxx
> CC: Brian Foster <bfoster@xxxxxxxxxx>
> CC: Vlastimil Babka <vbabka@xxxxxxx>
> Reported-by: Petr T�ma <petr.tuma@xxxxxxxxxxxxxxx>
> Fixes: 99579ccec4e271c3d4d4e7c946058766812afdab
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>
>
> So, yeah, we reverted the fix for a crash rather than trying to fix
> the adverse behaviour caused by invalidation of a dirty page.
>
> e.g. why didn't we simply clear the PageDirty flag in
> xfs_vm_invalidatepage()? The page is being invalidated - it's
> contents will never get written back - so having delalloc or
> unwritten extents over that page at the time it is invalidated is a
> bug and the original fix would have triggered warnings about
> this....
Seems like a reasonable revert/change, but given that ext3 was killed
off long ago, is it even still the case that the mm can feed releasepage
a dirty clean page? If that is the case, then isn't it time to fix the
mm too?
--D
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
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