linux-ext4 - Re: [PATCH v2 2/2] ext4: update i_disksize if direct write past ondisk size

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Message-ID: <20180322154548.GI2852@thunk.org>
Date:   Thu, 22 Mar 2018 11:45:48 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Eryu Guan <guaneryu@...il.com>
Cc:     linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v2 2/2] ext4: update i_disksize if direct write past
 ondisk size

On Mon, Mar 12, 2018 at 11:21:56PM +0800, Eryu Guan wrote:
> Currently in ext4 direct write path, we update i_disksize only when
> new eof is greater than i_size, and don't update it even when new
> eof is greater than i_disksize but less than i_size. This doesn't
> work well with delalloc buffer write, which updates i_size and
> i_disksize only when delalloc blocks are resolved (at writeback
> time), the i_disksize from direct write can be lost if a previous
> buffer write succeeded at write time but failed at writeback time,
> then results in corrupted ondisk inode size.
> 
> Consider this case, first buffer write 4k data to a new file at
> offset 16k with delayed allocation, then direct write 4k data to the
> same file at offset 4k before delalloc blocks are resolved, which
> doesn't update i_disksize because it writes within i_size(20k), but
> the extent tree metadata has been committed in journal. Then
> writeback of the delalloc blocks fails (due to device error etc.),
> and i_size/i_disksize from buffer write can't be written to disk
> (still zero). A subsequent umount/mount cycle recovers journal and
> writes extent tree metadata from direct write to disk, but with
> i_disksize being zero.
> 
> Fix it by updating i_disksize too in direct write path when new eof
> is greater than i_disksize but less than i_size, so i_disksize is
> always consistent with direct write.
> 
> This fixes occasional i_size corruption in fstests generic/475.
> 
> Signed-off-by: Eryu Guan <guaneryu@...il.com>

Updated, thanks.

					- Ted