linux-ext4 - Re: [PATCH 3/3] filemap: don't call generic_write

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Date:	Wed, 08 Feb 2012 11:38:22 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: [PATCH 3/3] filemap: don't call generic_write_sync for -EIOCBQUEUED

Jan Kara <jack@...e.cz> writes:

> On Tue 07-02-12 15:39:06, Jeff Moyer wrote:
>> Christoph Hellwig <hch@...radead.org> writes:
>> > On Mon, Feb 06, 2012 at 11:33:29AM -0500, Jeff Moyer wrote:
>> >> > code, right? Before that we'd drain the IO queue when cache flush is issued
>> >> > and thus effectively wait for IO completion...
>> >> 
>> >> Right, though hch seems to think even then the problem existed.
>> >
>> > I was wrong, using -o barrier it didn't.  That was however not something
>> > people using O_SYNC heavy production loads would do, they'd use disabled
>> > caches and nobarrier.
>> >
>> >> > Also I was thinking whether we couldn't implement the fix in VFS. Basically
>> >> > it would be the same like the fix for ext4. Like having a per-sb workqueue
>> >> > and queue work calling generic_write_sync() from end_io handler when the
>> >> > file is O_SYNC? That would solve the issue for all filesystems...
>> >> 
>> >> Well, that would require buy-in from the other file system developers.
>> >> What do the XFS folks think?
>> >
>> > I don't think using that code for XFS makes sene.  But just like
>> > generic_write_sync there's no reason it can't be added to generic code,
>> > just make sure only generic_file_aio_write/__generic_file_aio_write use
>> > it, but generic_file_buffered_write and generic_file_direct_write stay
>> > clear of it.
>> 
>> ext4_file_write (ext4's .aio_write routine) calls into
>> generic_file_aio_write.  So, I don't think we can generalize that this
>> routine means that the file system doesn't install its own endio
>> handler.  What's more, we'd have to pass an endio routine down the call
>> stack quite a ways.  In all, I think that would be an uglier solution to
>> the problem.  Did I miss something?
>   I think it can be done in a relatively elegant way. POC patch (completely
> untested) is attached. What do you think? All filesystems using
> blockdev_direct_IO() can be easily converted to use this, gfs2 & ocfs2 can
> also use the framework. That leaves only ext4, xfs & btrfs which need
> special handling. Actually, maybe btrfs could be converted as well because
> it doesn't seem to need to offload anything else to workqueue. But I'm not
> really sure...

I like it!

-Jeff
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