linux-kernel - Re: [PATCH] fixlet: Remove fs

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Message-ID: <CAN5ZDdfVEvxgwmRPwMVSq7iqTTaQ5fi5A-qtCsiqzTRjhOuL+A@mail.gmail.com>
Date:	Mon, 11 Jul 2011 13:44:22 -0700
From:	Justin TerAvest <teravest@...gle.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	jaxboe@...ionio.com, viro@...iv.linux.org.uk, jeffm@...e.de,
	tytso@...gle.com, mingo@...e.hu, peterz@...radead.org,
	linux-kernel@...r.kernel.org, reiserfs-devel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fixlet: Remove fs_excl from struct task.

Jens,

Have you had a chance to look at this? Christoph mentioned you'd
wanted to keep this,
but the last discussion I see is from 2009 and it doesn't look like
much has happened since then.


On Fri, Jun 24, 2011 at 3:02 PM, Christoph Hellwig <hch@...radead.org> wrote:
> On Fri, Jun 24, 2011 at 01:58:27PM -0700, Justin TerAvest wrote:
>> fs_excl is a poor man's priority inheritance for filesystems to hint to
>> the block layer that an operation is important. It was never clearly
>> specified, not widely adopted, and will not prevent starvation in many
>> cases (like across cgroups).
>>
>> I talked to Ted Ts'o about this, and he said that it used to used more
>> frequently in the 2.4 and prior versions of Linux, back when we were
>> first converting from the Big Kernel Lock to having subsystem level
>> locks, and so it made sense to use fs_excl when a process owned the
>> global fs mutex and was waiting for an I/O to complete, but it's no
>> longer used much at all, and filesystems have better ways to mark an I/O
>> request as high priority.
>
> That's not quite true, it was added in Linux 2.6.13 in commit
> 22e2c507c301c3dbbcf91b4948b88f78842ee6c9:
>
>        [PATCH] Update cfq io scheduler to time sliced design
>
> The users back then where the same as today: a few reiserfs journal
> callsites and lock_super.  In addition to the lock_super uses in various
> fringe filesystems still left today it was also used around ->put_super
> (aka umount) and ->write_super, which at the point had already lost the
> grunt work of sync action to ->sync_fs.
>
> That beeing said I never liked it and asked for a removal a while ago,
> but Jens still wanted to keep it.  As far as I'm concerned we should
> kill it gently, and if any regressions arise fix them with a bio flag.
>
>
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