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

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Message-ID: <20141215221100.GA4637@mew>
Date:	Mon, 15 Dec 2014 14:11:00 -0800
From:	Omar Sandoval <osandov@...ndov.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>,
	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 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.
>
Ok, I got the swap code working with ->read_iter/->write_iter without
too much trouble. I wanted to double check before I submit if there's
any gotchas involved with adding the O_DIRECT flag to a file pointer
after it has been opened -- swapon opens the swapfile before we know if
we're using the SWP_FILE infrastructure, and we need to add O_DIRECT so
->{read,write}_iter use direct I/O, but we can't add O_DIRECT to the
original open without excluding filesystems that support the old bmap
path but not direct I/O.

-- 
Omar
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