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Date: Tue, 13 Mar 2018 00:14:40 -0700
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: fallocate on XFS for swap
Message-ID: <20180313071440.GA23797@infradead.org>
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To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>, "Darrick J. Wong" <darrick.wong@oracle.com>, "Besogonov, Aleksei" <cyberax@amazon.com>, "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, xfs <linux-xfs@vger.kernel.org>

On Tue, Mar 13, 2018 at 08:46:26AM +1100, Dave Chinner wrote:
> > So maybe we want a layout based swap code instead of reinventing it,
> > with the slight twist to the layout break code to never try a lease
> > break and just return an error for the IS_SWAPFILE case.
> 
> Hmmm - won't that change user visible behaviour on swapfiles? Not
> that it would be a bad thing to reject read/write from root on swap
> files, but it would make XFS different to everything else.

We already can't writew to active swap files, thank god:

root@testvm:~# dd if=/dev/zero of=swapfile bs=1M count=64
64+0 records in
64+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 0.0458446 s, 1.5 GB/s
mkswap swapfile
mkswap: swapfile: insecure permissions 0644, 0600 suggested.
Setting up swapspace version 1, size = 64 MiB (67104768 bytes)
no label, UUID=bb42b883-f224-4627-8580-c1ba9f4569ab
root@testvm:~# swapon swapfile
swapon: /root/swapfile: insecure permissions 0644, 0600 suggested.
[   54.165439] Adding 65532k swap on /root/swapfile.  Priority:-2 extents:1 across:65532k
root@testvm:~# dd if=/dev/zero of=swapfile bs=1M count=64
dd: failed to open 'swapfile': Text file busy

> 
> Speaking of which - we probably need to spend some time at LSFMM in
> the fs track talking about the iomap infrastructure and long term
> plans to migrate the major filesystems to it....

I won't be there, as I'll be busy working the local election ballot.