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Re: [PATCH 0/2 v2] Fix O_SYNC AIO DIO

To: Jan Kara <jack@xxxxxxx>
Subject: Re: [PATCH 0/2 v2] Fix O_SYNC AIO DIO
From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date: Fri, 30 Aug 2013 16:53:01 +0100
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, Christoph Hellwig <hch@xxxxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1376471456-11966-1-git-send-email-jack@xxxxxxx>
References: <1376471456-11966-1-git-send-email-jack@xxxxxxx>
Sender: Al Viro <viro@xxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Aug 14, 2013 at 11:10:54AM +0200, Jan Kara wrote:
>   Hello,
> 
>   this is second iteration of patches to fix handling of O_SYNC AIO DIO.
> Since previous version I've addressed Dave's comments:
>  - slightly expanded changelog of the first patch
>  - workqueue is now created with parameters allowing paralelism
>  - workqueue name contains sb->s_id
>  - workqueue is created on demand (I decided to do this to reduce the overhead
>    in unnecessary cases)
> 
> The patchset survives xfstests run for ext4 & xfs so it should be sane. Since
> this touches several filesystems (although only ext4 & xfs are non-trivial),
> the question is who should carry these patches. Maybe Al? But since xfs and
> ext4 changes are non-trivial, I'd like to have a review from their
> developers...

Looks sane, except that I'd probably put destroying the queue after
evict_inodes(), next to ->put_super() call.

Said that, there's another interesting problem in the code affected by that
sucker: generic_file_aio_write() might very well sync the wrong range.
Consider O_APPEND case; __generic_file_aio_write() will call
generic_write_checks(), which will update its copy of pos, and proceed to
write starting from there.  All right and proper, but then we return into
generic_file_aio_write() and sync the range of the right length, starting
at the *original* value of pos...

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