fallocate on XFS for swap

["Besogonov, Aleksei" <cyberax@amazon.com>]

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 .

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.
2. Change generic_swap_activate to use a more modern interface, by adding fiemap-like operation to address_space_operations with fallback on bmap().
3. Add an XFS-specific implementation of swapfile_activate.

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.