linux-ext4 - fsx-linux loosing mmap() writes under memory pressure

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Date:	Wed, 4 Mar 2009 15:51:09 +0100
From:	Jan Kara <jack@...e.cz>
To:	linux-kernel@...r.kernel.org
Cc:	user-mode-linux-devel@...ts.sourceforge.net,
	linux-ext4@...r.kernel.org
Subject: fsx-linux loosing mmap() writes under memory pressure

  Hello,

  first, I'd like to point out that this has happened under UML so it can
be just some obscure bug in that architecture but I belive it's worth
debugging anyway. Now to the problem:
  This has happened with today Linus's git snapshot. The filesystem is ext3
with *1KB* blocksize. I booted UML with 64MB of memory and run (these are
test's from Andrew Morton's torture tests):
  fsx-linux -l 8000000 /mnt/testfile
  bash-shared-mapping -t 8 /mnt/bashfile 50000000
(the second test just makes the UML under memory pressure and stresses the
filesystem, otherwise it does not interact with fsx-linux in any way).
  After some time (like an hour) fsx-linux reported the file is corrupted. I
tried again and it happened again so probably some debugging should be
possible.
  Both times it seems we've simply completely lost a write which happened
through mmap (2 pages in the first case, 3 pages in the second case). Also
I've checked and in the first case no blocks are allocated for the offsets
where the data should be so most probably we've lost the write before
block_write_full_page() called get_block().
  I'll debug this further but I wanted let people know there's some problem
and maybe somebody has some bright idea :).  I'm attaching the log from fsx
if someone is interested. 

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR

View attachment "testfile.fsxlog" of type "text/plain" (67661 bytes)