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Date: Thu, 3 May 2018 14:26:20 -0700
From: Omar Sandoval <osandov@osandov.com>
Subject: Re: [PATCH v3 1/2] iomap: add a swapfile activation function
Message-ID: <20180503212620.GB26037@vader>
References: <20180503174659.GD4127@magnolia>
 <20180503205803.GA26037@vader>
 <20180503212447.GG4141@magnolia>
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To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: xfs <linux-xfs@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, linux-mm@kvack.org, hch@infradead.org, cyberax@amazon.com, jack@suse.cz, Eryu Guan <guaneryu@gmail.com>

On Thu, May 03, 2018 at 02:24:47PM -0700, Darrick J. Wong wrote:
> On Thu, May 03, 2018 at 01:58:03PM -0700, Omar Sandoval wrote:
> > On Thu, May 03, 2018 at 10:46:59AM -0700, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > > 
> > > Add a new iomap_swapfile_activate function so that filesystems can
> > > activate swap files without having to use the obsolete and slow bmap
> > > function.  This enables XFS to support fallocate'd swap files and
> > > swap files on realtime devices.
> > > 
> > 
> > Shouldn't we also prevent the extents of an active swapfile from
> > becoming shared? If I swapon(a) and reflink(a, b), swapout to a now has
> > to break the reflink or corrupt b! In my old Btrfs swapfile series [1] I
> > just forbid all reflink operations on active swapfiles. 
> 
> xfs already does this in its reflink handler: it takes the inode lock &
> bails out if IS_SWAPFILE().  swapon calls claim_swapfile to take the
> inode lock and sets S_SWAPFILE (if successful) so the two are
> effectively locked out from each other...
> 
> > One thing to note is that then this will need a matching
> > ->swap_deactivate(), which currently isn't called if ->swap_activate()
> > returned > 0.
> 
> ...so there shouldn't be any state to undo if the
> iomap_swapfile_activate fails.

Makes sense, thanks.