Re: [RFC][PATCH v3 0/9] mm: Use DIO for swap and fix NFS swapfiles

[David Sterba]

On Fri, Sep 24, 2021 at 06:17:52PM +0100, David Howells wrote:
> 
> Hi Willy, Trond, Christoph,
> 
> Here's v3 of a change to make reads and writes from the swapfile use async
> DIO, adding a new ->swap_rw() address_space method, rather than readpage()
> or direct_IO(), as requested by Willy.  This allows NFS to bypass the write
> checks that prevent swapfiles from working, plus a bunch of other checks
> that may or may not be necessary.
> 
> Whilst trying to make this work, I found that NFS's support for swapfiles
> seems to have been non-functional since Aug 2019 (I think), so the first
> patch fixes that.  Question is: do we actually *want* to keep this
> functionality, given that it seems that no one's tested it with an upstream
> kernel in the last couple of years?
> 
> 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.
> 
> The synchronous DIO I/O code on NFS, raw blockdev, ext4 swapfile and xfs
> swapfile all seem to work fine.  Btrfs refuses to swapon because the file
> might be CoW'd.  I've tried doing "chattr +C", but that didn't help.

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.