linux-kernel - Re: [RFC PATCH 1/2] mm, vmscan: account the number of isolated pages per zone

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Date:   Mon, 6 Feb 2017 09:35:33 -0500
From:   Brian Foster <bfoster@...hat.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     mhocko@...nel.org, david@...morbit.com, dchinner@...hat.com,
        hch@....de, mgorman@...e.de, viro@...IV.linux.org.uk,
        linux-mm@...ck.org, hannes@...xchg.org,
        linux-kernel@...r.kernel.org, darrick.wong@...cle.com,
        linux-xfs@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] mm, vmscan: account the number of isolated pages
 per zone

On Mon, Feb 06, 2017 at 03:29:24PM +0900, Tetsuo Handa wrote:
> Brian Foster wrote:
> > On Fri, Feb 03, 2017 at 03:50:09PM +0100, Michal Hocko wrote:
> > > [Let's CC more xfs people]
> > > 
> > > On Fri 03-02-17 19:57:39, Tetsuo Handa wrote:
> > > [...]
> > > > (1) I got an assertion failure.
> > > 
> > > I suspect this is a result of
> > > http://lkml.kernel.org/r/20170201092706.9966-2-mhocko@kernel.org
> > > I have no idea what the assert means though.
> > > 
> > > > 
> > > > [  969.626518] Killed process 6262 (oom-write) total-vm:2166856kB, anon-rss:1128732kB, file-rss:4kB, shmem-rss:0kB
> > > > [  969.958307] oom_reaper: reaped process 6262 (oom-write), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> > > > [  972.114644] XFS: Assertion failed: oldlen > newlen, file: fs/xfs/libxfs/xfs_bmap.c, line: 2867
> > 
> > Indirect block reservation underrun on delayed allocation extent merge.
> > These are extra blocks are used for the inode bmap btree when a delalloc
> > extent is converted to physical blocks. We're in a case where we expect
> > to only ever free excess blocks due to a merge of extents with
> > independent reservations, but a situation occurs where we actually need
> > blocks and hence the assert fails. This can occur if an extent is merged
> > with one that has a reservation less than the expected worst case
> > reservation for its size (due to previous extent splits due to hole
> > punches, for example). Therefore, I think the core expectation that
> > xfs_bmap_add_extent_hole_delay() will always have enough blocks
> > pre-reserved is invalid.
> > 
> > Can you describe the workload that reproduces this? FWIW, I think the
> > way xfs_bmap_add_extent_hole_delay() currently works is likely broken
> > and have a couple patches to fix up indlen reservation that I haven't
> > posted yet. The diff that deals with this particular bit is appended.
> > Care to give that a try?
> 
> The workload is to write to a single file on XFS from 10 processes demonstrated at
> http://lkml.kernel.org/r/201512052133.IAE00551.LSOQFtMFFVOHOJ@I-love.SAKURA.ne.jp
> using "while :; do ./oom-write; done" loop on a VM with 4CPUs / 2048MB RAM.
> With this XFS_FILBLKS_MIN() change applied, I no longer hit assertion failures.
> 

Thanks for testing. Well, that's an interesting workload. I couldn't
reproduce on a few quick tries in a similarly configured vm.

Normally I'd expect to see this kind of thing on a hole punching
workload or dealing with large, sparse files that make use of
speculative preallocation (post-eof blocks allocated in anticipation of
file extending writes). I'm wondering if what is happening here is that
the appending writes and file closes due to oom kills are generating
speculative preallocs and prealloc truncates, respectively, and that
causes prealloc extents at the eof boundary to be split up and then
re-merged by surviving appending writers.

/tmp/file _is_ on an XFS filesystem in your test, correct? If so and if
you still have the output file from a test that reproduced, could you
get the 'xfs_io -c "fiemap -v" <file>' output?

I suppose another possibility is that prealloc occurs, write failure(s)
leads to extent splits via unmapping the target range of the write, and
then surviving writers generate the warning on a delalloc extent merge..

Brian

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