[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
