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 15:29:24 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     bfoster@...hat.com, mhocko@...nel.org
Cc:     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

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.