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Date:   Fri, 1 Jul 2022 12:03:23 -0400
From:   Brian Foster <bfoster@redhat.com>
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     Dave Chinner <david@fromorbit.com>,
        Matthew Wilcox <willy@infradead.org>,
        linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        linux-mm@kvack.org
Subject: Re: Multi-page folio issues in 5.19-rc4 (was [PATCH v3 25/25] xfs:
 Support large folios)
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On Wed, Jun 29, 2022 at 01:22:06PM -0700, Darrick J. Wong wrote:
> On Wed, Jun 29, 2022 at 08:57:30AM -0400, Brian Foster wrote:
> > On Tue, Jun 28, 2022 at 04:21:55PM -0700, Darrick J. Wong wrote:
> > > On Wed, Jun 29, 2022 at 08:17:57AM +1000, Dave Chinner wrote:
> > > > On Tue, Jun 28, 2022 at 02:18:24PM +0100, Matthew Wilcox wrote:
> > > > > On Tue, Jun 28, 2022 at 12:31:55PM +0100, Matthew Wilcox wrote:
> > > > > > On Tue, Jun 28, 2022 at 12:27:40PM +0100, Matthew Wilcox wrote:
> > > > > > > On Tue, Jun 28, 2022 at 05:31:20PM +1000, Dave Chinner wrote:
> > > > > > > > So using this technique, I've discovered that there's a dirty page
> > > > > > > > accounting leak that eventually results in fsx hanging in
> > > > > > > > balance_dirty_pages().
> > > > > > > 
> > > > > > > Alas, I think this is only an accounting error, and not related to
> > > > > > > the problem(s) that Darrick & Zorro are seeing.  I think what you're
> > > > > > > seeing is dirty pages being dropped at truncation without the
> > > > > > > appropriate accounting.  ie this should be the fix:
> > > > > > 
> > > > > > Argh, try one that actually compiles.
> > > > > 
> > > > > ... that one's going to underflow the accounting.  Maybe I shouldn't
> > > > > be writing code at 6am?
> > > > > 
> > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > > > index f7248002dad9..4eec6ee83e44 100644
> > > > > --- a/mm/huge_memory.c
> > > > > +++ b/mm/huge_memory.c
> > > > > @@ -18,6 +18,7 @@
> > > > >  #include <linux/shrinker.h>
> > > > >  #include <linux/mm_inline.h>
> > > > >  #include <linux/swapops.h>
> > > > > +#include <linux/backing-dev.h>
> > > > >  #include <linux/dax.h>
> > > > >  #include <linux/khugepaged.h>
> > > > >  #include <linux/freezer.h>
> > > > > @@ -2439,11 +2440,15 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> > > > >  		__split_huge_page_tail(head, i, lruvec, list);
> > > > >  		/* Some pages can be beyond EOF: drop them from page cache */
> > > > >  		if (head[i].index >= end) {
> > > > > -			ClearPageDirty(head + i);
> > > > > -			__delete_from_page_cache(head + i, NULL);
> > > > > +			struct folio *tail = page_folio(head + i);
> > > > > +
> > > > >  			if (shmem_mapping(head->mapping))
> > > > >  				shmem_uncharge(head->mapping->host, 1);
> > > > > -			put_page(head + i);
> > > > > +			else if (folio_test_clear_dirty(tail))
> > > > > +				folio_account_cleaned(tail,
> > > > > +					inode_to_wb(folio->mapping->host));
> > > > > +			__filemap_remove_folio(tail, NULL);
> > > > > +			folio_put(tail);
> > > > >  		} else if (!PageAnon(page)) {
> > > > >  			__xa_store(&head->mapping->i_pages, head[i].index,
> > > > >  					head + i, 0);
> > > > > 
> > > > 
> > > > Yup, that fixes the leak.
> > > > 
> > > > Tested-by: Dave Chinner <dchinner@redhat.com>
> > > 
> > > Four hours of generic/522 running is long enough to conclude that this
> > > is likely the fix for my problem and migrate long soak testing to my
> > > main g/522 rig and:
> > > 
> > > Tested-by: Darrick J. Wong <djwong@kernel.org>
> > > 
> > 
> > Just based on Willy's earlier comment.. what I would probably be a
> > little careful/curious about here is whether the accounting fix leads to
> > an indirect behavior change that does impact reproducibility of the
> > corruption problem. For example, does artificially escalated dirty page
> > tracking lead to increased reclaim/writeback activity than might
> > otherwise occur, and thus contend with the fs workload? Clearly it has
> > some impact based on Dave's balance_dirty_pages() problem reproducer,
> > but I don't know if it extends beyond that off the top of my head. That
> > might make some sense if the workload is fsx, since that doesn't
> > typically stress cache/memory usage the way a large fsstress workload or
> > something might.
> > 
> > So for example, interesting questions might be... Do your corruption
> > events happen to correspond with dirty page accounting crossing some
> > threshold based on available memory in your test environment? Does
> > reducing available memory affect reproducibility? Etc.
> 
> Yeah, I wonder that too now.  I managed to trace generic/522 a couple of
> times before willy's patch dropped.  From what I could tell, a large
> folio X would get page P assigned to the fsx file's page cache to cover
> range R, dirtied, and written to disk.  At some point later, we'd
> reflink into part of the file range adjacent to P, but not P itself.
> I /think/ that should have caused the whole folio to get invalidated?
> 
> Then some more things happened (none of which dirtied R, according to
> fsx) and then suddenly writeback would trigger on some page (don't know
> which) that would write to the disk blocks backing R.  I'm fairly sure
> that's where the incorrect disk contents came from.
> 
> Next, we'd reflink part of the file range including R into a different
> part of the file (call it R2).  fsx would read R2, bringing a new page
> into cache, and it wouldn't match the fsxgood buffer, leading to fsx
> aborting.
> 
> After a umount/mount cycle, reading R and R2 would both reveal the
> incorrect contents that had caused fsx to abort.
> 

FWIW, I hadn't been able to reproduce this in my default environment to
this point. With the memory leak issue in the light, I was eventually
able to by reducing dirty_bytes to something the system would be more
likely to hit sooner (i.e. 16-32MB), but I also see stalling behavior
and whatnot due to the leak that requires backing off from the specified
dirty limit every so often.

If I apply the accounting patch to avoid the leak and set
dirty_background_bytes to something notably aggressive (1kB), the test
survived 100 iterations or so before I stopped it. If I then set
dirty_bytes to something similarly aggressive (1MB), I hit the failure
on the next iteration (assuming it's the same problem). It's spinning
again at ~25 or so iterations without a failure so far, so I'd have to
wait and see how reliable the reproducer really is. Though if it doesn't
reoccur soonish, perhaps I'll try reducing dirty_bytes a bit more...

My suspicion based on these characteristics would be that the blocking
limit triggers more aggressive reclaim/invalidation, and thus helps
detect the problem sooner. If reflink is involved purely as a cache
invalidation step (i.e. so a subsequent read will hit the disk and
detect a cache inconsistency), then it might be interesting to see if it
can still be reproduced without reflink operations enabled but instead
with some combination of the -f/-X fsx flags to perform more flush
invals and on-disk data checks..

Brian

> Unfortunately the second ftrace attempt ate some trace data, so I was
> unable to figure out if the same thing happened again.
> 
> At this point I really need to get on reviewing patches for 5.20, so
> I'll try to keep poking at this (examining the trace data requires a lot
> of concentration which isn't really possible while sawzall construction
> is going on at home) but at worst I can ask Linus to merge a patch for
> 5.19 final that makes setting mapping_set_large_folio a
> Kconfig/CONFIG_XFS_DEBUG option.
> 
> --D
> 
> > 
> > Brian
> > 
> > > --D
> > > 
> > > > Cheers,
> > > > 
> > > > Dave.
> > > > -- 
> > > > Dave Chinner
> > > > david@fromorbit.com
> > > 
> > 
>