linux-kernel - Re: get_fs_excl/put_fs_excl/has_fs

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Message-ID: <20090424184047.GA17001@lst.de>
Date:	Fri, 24 Apr 2009 20:40:47 +0200
From:	Christoph Hellwig <hch@....de>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Christoph Hellwig <hch@....de>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: get_fs_excl/put_fs_excl/has_fs_excl

On Thu, Apr 23, 2009 at 09:21:24PM +0200, Jens Axboe wrote:
> The intent was to add some sort of notification mechanism from the file
> system to inform the IO scheduler (and others?) that this process is how
> holding a file system wide resource. So if you have a low priority
> process getting access to such a resource, you want to boost its
> priority to avoid higher priority apps getting stuck beind it. Sort of a
> poor mans priority inheritance.
> 
> It would be wonderful if you could kick this process more into gear on
> the fs side...

So what are the calls in lock_super/unlock_super supposed to be for?
->write_super?  While that can sync bits out most of the heavy lifting
is now done in ->sync_fs for most filesystems.  ->remount_fs?  This is
going to block all other I/O anyway.  ->put_super?  Surely not :)

ext3/4 internal bits?  Doesn't seem to be used for any journal related
activity but mostly as protection against resizing (the whole lock_super
usage in ext3/4 looks odd to me, interestingly there's none at all in
ext2.  Maybe someone of the extN crowd should audit and get rid of it in
favour of a better fs-specific lock)

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