disable large folios for shmem file used by xfs xfile

[Joonas Lahtinen]

Quoting Matthew Wilcox (2024-01-10 14:37:18)
> On Wed, Jan 10, 2024 at 10:21:07AM +0100, Christoph Hellwig wrote:
> > Hi all,
> > 
> > Darrick reported that the fairly new XFS xfile code blows up when force
> > enabling large folio for shmem.  This series fixes this quickly by disabling
> > large folios for this particular shmem file for now until it can be fixed
> > properly, which will be a lot more invasive.
> > 
> > I've added most of you to the CC list as I suspect most other users of
> > shmem_file_setup and friends will have similar issues.
> 
> The graphics users _want_ to use large folios.  I'm pretty sure they've
> been tested with this.

Correct. We've done quite a bit of optimization in userspace and
enabling in kernel to take advantage of page sizes of 2M and beyond.

However we specifically pass "huge=within_size" to vfs_kern_mount when
creating a private mount of tmpfs for the purpose of i915 created
allocations.

Older hardware also had some address hashing bugs where 2M aligned
memory caused a lot of collisions in TLB so we don't enable it always.

You can see drivers/gpu/drm/i915/gem/i915_gemfs.c function
i915_gemfs_init for details and references.

So in short, functionality wise we should be fine either default
for using 2M pages or not. If they become the default, we'd probably
want an option that would still be able to prevent them for performance
regression reasons on older hardware.

Regards, Joonas

> It's just XFS that didn't know about this
> feature of shmem.