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From:   David Howells <dhowells@redhat.com>
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To:     dsterba@suse.cz
Cc:     dhowells@redhat.com
Cc:     willy@infradead.org, Chris Mason <clm@fb.com>,
        linux-block@vger.kernel.org, ceph-devel@vger.kernel.org,
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        linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-cifs@vger.kernel.org, linux-nfs@vger.kernel.org,
        Ilya Dryomov <idryomov@gmail.com>, linux-btrfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH v3 0/9] mm: Use DIO for swap and fix NFS swapfiles
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Date:   Wed, 29 Sep 2021 16:45:02 +0100
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David Sterba <dsterba@suse.cz> wrote:

> > There are additional patches to get rid of noop_direct_IO and replace =
it
> > with a feature bitmask, to make btrfs, ext4, xfs and raw blockdevs use=
 the
> > new ->swap_rw method and thence remove the direct BIO submission paths=
 from
> > swap.
> > =

> > I kept the IOCB_SWAP flag, using it to enable REQ_SWAP.  I'm not sure =
if
> > that's necessary, but it seems accounting related.
>
> There was probably some step missing. The file must not have holes, so
> either do 'dd' to the right size or use fallocate (which is recommended
> in manual page btrfs(5) SWAPFILE SUPPORT). There are some fstests
> exercising swapfile (grep -l _format_swapfile tests/generic/*) so you
> could try that without having to set up the swapfile manually.

Yeah.  As advised elsewhere, I removed the file and recreated it, doing th=
e
chattr before extending the file.  At that point swapon worked.  It didn't
work though, and various userspace programs started dying.  I'm guessing m=
y
btrfs_swap_rw() is wrong somehow.

David