Re: fallocate on XFS for swap

["Darrick J. Wong" <darrick.wong@oracle.com>]

[you really ought to cc the xfs list]

On Fri, Mar 09, 2018 at 10:05:24PM +0000, Besogonov, Aleksei wrote:
> Hi!
> 
> We’re working at Amazon on making XFS our default root filesystem for
> the upcoming Amazon Linux 2 (now in prod preview). One of the problems
> that we’ve encountered is inability to use fallocated files for swap
> on XFS. This is really important for us, since we’re shipping our
> current Amazon Linux with hibernation support .

<shudder>

> I’ve traced the problem to bmap(), used in generic_swapfile_activate
> call, which returns 0 for blocks inside holes created by fallocate and
> Dave Chinner confirmed it in a private email. I’m thinking about ways
> to fix it, so far I see the following possibilities:
> 
> 1. Change bmap() to not return zeroes for blocks inside holes. But
> this is an ABI change and it likely will break some obscure userspace
> utility somewhere.

bmap is a horrible interface, let's leave it to wither and eventually go
away.

> 2. Change generic_swap_activate to use a more modern interface, by
> adding fiemap-like operation to address_space_operations with fallback
> on bmap().

Probably the best idea, but see fs/iomap.c since we're basically leasing
a chunk of file space to the kernel.  Leasing space to a user that wants
direct access is becoming rather common (rdma, map_sync, etc.)

> 3. Add an XFS-specific implementation of swapfile_activate.

Ugh no.

> What do the people think about it? I kinda like option 2, since it'll
> make fallocate() work for any other FS that implements fiemap.

--D