linux-ext4 - Re: [PATCH 1/2] ext4: avoid hangs in ext4_da_should_update_i

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Date:	Tue, 13 Dec 2011 21:44:35 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 1/2] ext4: avoid hangs in
 ext4_da_should_update_i_disksize()

On Mon, Dec 12, 2011 at 03:27:07AM +0100, Andrea Arcangeli wrote:
> If the pte mapping in generic_perform_write() is unmapped between
> iov_iter_fault_in_readable() and iov_iter_copy_from_user_atomic(), the
> "copied" parameter to ->end_write can be zero. ext4 couldn't cope with
> it with delayed allocations enabled. This skips the i_disksize
> enlargement logic if copied is zero and no new data was appeneded to
> the inode.
      ...
> 
> This is 100% reproducible with some autonuma development code tuned in
> a very aggressive manner (not normal way even for knumad) which does
> "exotic" changes to the ptes. It wouldn't normally trigger but I don't
> see why it can't happen normally if the page is added to swap cache in
> between the two faults leading to "copied" being zero (which then
> hangs in ext4). So it should be fixed. Especially possible with lumpy
> reclaim (albeit disabled if compaction is enabled) as that would
> ignore the young bits in the ptes.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>

Thanks, applied.

						- Ted
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