linux-kernel - Re: fsx-linux loosing mmap() writes under memory pressure

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Date:	Wed, 4 Mar 2009 16:55:35 +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: Re: fsx-linux loosing mmap() writes under memory pressure

On Wed 04-03-09 15:51:09, Jan Kara wrote:
>   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. 
  Testing a bit more, I managed to reproduce the problem on ext2 and what's
more strange, now the lost page was written via ordinary write() (fsxlog
attached). So I believe this is more likely to be UML specific...

									Honza

View attachment "testfile.fsxlog" of type "text/plain" (66907 bytes)