Date:   Wed, 17 Aug 2022 19:36:27 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Brian Foster <bfoster@...hat.com>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        linux-mm@...ck.org
Subject: Re: Multi-page folio issues in 5.19-rc4 (was [PATCH v3 25/25] xfs:
 Support large folios)

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@...hat.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@...nel.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?

Yeah, I think you're right, Brian. There is an underlying problem
here, and the writeback/memory pressure bugs exposed it for everyone
to see.  IOWs, we do indeed have a pre-existing problem with partial
writes, iomap, unwritten extents, writeback and memory reclaim.

(Please s/page/folio/ rather than complain I'm calling things pages
and not folios, I'm juggling between RHEL-8 code and upstream
here and it's just easier to refer to everything as pages as I write
it.)

(I'm assuming people are familiar with the iomap buffered IO code
at this point.)

Let's do a partial write into a page. Let's say the second half of
the page. __iomap_begin_write() drops into iomap_adjust_read_range()
to determine the range that needs zeroing or reading from disk.  We
then call iomap_block_needs_zeroing() to determine if we zero -
which we do if it's a newly allocated or unwritten extent. Otherwise
we read it from disk.

Let's put this partial write over a large unwritten extent, which
means we zero the first part of the page, and that writing it back
runs unwritten extent conversion at IO completion. This changes the
extent backing this file offset from unwritten to written.

Then, add extreme memory pressure, so writeback is already running
and soon after the page has been dirtied it gets marked for
cleaning. Therefore, we have the possibility of background IO
submission and memory reclaim occurring at the same time as new
writes are occurring to the file.

To that end, let's add a large racing write that *ends* on this same
page that we have already partially filled with data - it will fill
the start of the page that currently contains zeros with real data.

Let's start that write() just before writeback of the already
uptodate partial page starts. iomap_apply will map the entire
write (and well beyond) as a single unwritten extent. This extent is
the same extent that the original page in the page cache already
covers, and this write would end by filling the remaining part of
that write.

While that second write is running around in the page cache
copy-in loop for the unwritten extent that backs that range, IO
submission runs and completes and converts the original partially
filled page to a written extent, then marks it clean.

[ Ayup, the extent map of the file just changed to be different
to the iomap that the current write() thinks the file layout has. ]

Further, because the partially written page is now clean,
memory reclaim can snaffle it up and it gets removed from the page
cache.

The second write finally gets to the last page of it's write and
doesn't find it in the page cache because memory reclaim removed it.
So it pulls a new page into the page cache in iomap_begin_write(),
and then in __iomap_begin_write we see that the we need to fill the
second half of the page with either zeros or data from disk.

We know that this data is on disk as the extent is now in written
state.  *However*, the cached iomap that we are currently using for
the write() says the range is -unwritten-, and so at that point
iomap_block_needs_zeroing() says "page needs zeroing". Hence
__iomap_begin_write zeroes the second half of the page instead of
reading it from disk, obliterating the data that the previous write
had already written there.

Ouchy, we have data corruption because the incoming write holds a
stale cached iomap......

At this point, I had a nasty feeling of deja vu.

Oh, yeah, we already infrastructure in place to avoid using stale
cached iomaps in ->writepages....

That is, iomap_writepage_map() calls ->map_block for every filesytem
block, even though the iomap_writepage_ctx has a cached iomap in it.
That's so the filesystem can check that the cached iomap is still
valid whilst the page we are writing back is held locked. This
protects writeback against races with truncate, hole punch, etc.

XFS does this via a call to xfs_imap_valid() - we keep a generation
number in the XFS extent trees that is bumped on every change so
that we can check whether a cached iomap is still valid. If the
generation numbers don't match, then we generate a new iomap for the
writepage context....

What is clear to me now is that we have the same stale cached iomap
problem with iomap_apply() because writeback can change the extent
map state in a way that influence incoming writes and those updates
are not in any way serialised against the incoming write IO path.

Hence we have to validate that the cached iomap is not stale before
we decide what to do with a new page in the page cache (zero or read
from disk), and we have to do this iomap validation when the newly
instantiated page is locked and ready for initialisation.

I have not thought past this point - I'm only *just* smart enough to
be able to dig out the root cause of this problem. No scratch that,
I'm not smart, just stubborn enough to ignore the fact I've got the
analysis wrong at least a dozen times already.

That said, this stale mapping problem existed long before we started
using unwritten extents for delayed allocation. The change to use
unwritten extents (to fix other data corruption issues) exposed us
to in-memory zeroing in this race condition case rather reading the
block of data from disk. We just hadn't armed the landmine....

I suspect we are going to need a new iomap validation operation, and
require the iomap actors to be able to handle iomap validation
failure.  What that looks like, I don't know as my brain
turned to goo and dribbled out my ears about 4 hours ago....

BTW, all the credit for finding this goes to Frank Sorenson - he did
all the hard work of reproducing the issue and narrowing down the
scope of the problem to a handful of XFS commits combined with
writeback and memory reclaim interactions. Without his hard work,
we'd still have no idea what was going on.....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com

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