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

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Date:	Sun, 21 Dec 2014 23:26:38 -0800
From:	Omar Sandoval <osandov@...ndov.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
	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:
> On Mon, Dec 15, 2014 at 08:56:15AM -0800, Christoph Hellwig wrote:
> > On Mon, Dec 15, 2014 at 05:27:05PM +0100, Jan Kara wrote:
> > > On Sun 14-12-14 21:26:56, Omar Sandoval wrote:
> > > > The generic write code locks i_mutex for a direct_IO. Swap-over-NFS
> > > > doesn't grab the mutex because nfs_direct_IO doesn't expect i_mutex to
> > > > be held, but most direct_IO implementations do.
> > >   I think you are speaking about direct IO writes only, aren't you? For DIO
> > > reads we don't hold i_mutex AFAICS. And also for DIO writes we don't
> > > necessarily hold i_mutex - see for example XFS which doesn't take i_mutex
> > > for direct IO writes. It uses it's internal rwlock for this (see
> > > xfs_file_dio_aio_write()). So I think this is just wrong.
> > 
> > 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...

Alright, now what? Using ->direct_IO directly is pretty much a no go
because of the different locking conventions as was pointed out. Maybe
some "no, really, just direct I/O" iocb flag?

-- 
Omar
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