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

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Date:	Wed, 17 Dec 2014 08:20:21 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Omar Sandoval <osandov@...ndov.com>, 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 12:06:10AM -0800, Christoph Hellwig wrote:

> > This seems to be more or less equivalent to doing a fcntl(F_SETFL) to
> > add the O_DIRECT flag to swap_file (which is a struct file *). Swapoff
> > calls filp_close on swap_file, so I don't see why it's necessary to
> > clear the flag.
> 
> filp_lose doesn't nessecarily destroy the file structure, there might be
> other reference to it, e.g. from dup() or descriptor passing.

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.
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