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

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Date:	Wed, 17 Dec 2014 06:58:32 -0800
From:	Omar Sandoval <osandov@...ndov.com>
To:	Christoph Hellwig <hch@...radead.org>,
	Al Viro <viro@...IV.linux.org.uk>
Cc:	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 Wed, Dec 17, 2014 at 12:24:37AM -0800, Christoph Hellwig wrote:
> On Wed, Dec 17, 2014 at 08:20:21AM +0000, Al Viro wrote:
> > Where the hell would those other references come from?  We open the damn
> > thing in sys_swapon(), never put it into descriptor tables, etc. and
> > the only reason why we use filp_close() instead of fput() is that we
> > would miss ->flush() otherwise.
> > 
> > Said that, why not simply *open* it with O_DIRECT to start with and be done
> > with that?  It's not as if those guys came preopened by caller - swapon(2)
> > gets a pathname and does opening itself.
> 
> Oops, should have dug deeper into the code.  For some reason I assumed
> the fd is passed in from userspace.
> 
> The suggestion from Al is much better, given that we never do normal
> I/O on the swapfile, just the bmap + direct bio submission which I hope
> could go away in favor of the direct I/O variant in the long run.

See my previous message. If we use O_DIRECT on the original open, then
filesystems that implement bmap but not direct_IO will no longer work.
These are the ones that I found in my tree:

adfs
befs
bfs
ecryptfs
efs
freevxfs
hpfs
isofs
minix
ntfs
omfs
qnx4
qnx6
sysv
ufs

Several of these are read only, and I can't imagine that anyone is using
a swapfile on any of the rest, but if someone is, this would be a
regression, wouldn't it?

-- 
Omar
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