Re: fallocate on XFS for swap

[Dave Chinner <david@fromorbit.com>]

On Fri, Mar 09, 2018 at 03:44:22PM -0800, Darrick J. Wong wrote:
> [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.)

thing is, we don't want in-kernel users of fiemap. We've got other
block mapping interfaces that can be used, such as iomap...

> > 3. Add an XFS-specific implementation of swapfile_activate.
> 
> Ugh no.

What we want is an iomap-based re-implementation of
generic_swap_activate(). One of the ways to plumb that in is to
use ->swapfile_activate() like so:

iomap_swapfile_activate()
{
	return iomap_apply(... iomap_swapfile_add_extent, ...)
}

xfs_vm_swapfile_activate()
{
	return iomap_swapfile_activate(xfs_iomap_ops);
}

	.swapfile_activate = xfs_vm_swapfile_activate()

And massage the swapfile_activate callout be friendly to fragmented
files. i.e. change the nfs caller to run a
"add_single_swap_extent()" caller rather than have to do it in the
generic code on return....

IOWs, I think the choices we have are to either re-implement
generic_swapfile_activate() and then be stuck with using get_block
style interfaces forever in XFS, or we use the filesystem specific
callout to implement more advanced generic support using the
filesystem supplied get_block/iomap interfaces for block mapping
like we do for everything else that the VM needs the filesystem to
do....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com