linux-kernel - Re: [PATCH 2/8] swap: lock i_mutex for swap_writepage direct

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Date:	Tue, 23 Dec 2014 01:37:10 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Jan Kara <jack@...e.cz>, Omar Sandoval <osandov@...ndov.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	David Sterba <dsterba@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/8] swap: lock i_mutex for swap_writepage direct_IO

On Sat, Dec 20, 2014 at 06:51:33AM +0000, Al Viro wrote:
> > The problem is that the use of ->direct_IO by the swap code is a gross
> > layering violation.  ->direct_IO is a callback for the filesystem, and
> > the swap code need to call ->read_iter instead of ->readpage and
> > ->write_tier instead of ->direct_IO, and leave the locking to the
> > filesystem.
> 
> The thing is, ->read_iter() and ->write_iter() might decide to fall back to 
> buffered IO path.  XFS is unusual in that respect - there O_DIRECT ends up
> with short write in such case.  Other filesystems, OTOH...

We'll just need a ->swap_activate method that makes sure we really do
direct I/O.  For all filesystems currently suporting swap just checking
that all blocks are allocated (as the ->bmap path already does) should
be enough.
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