disable large folios for shmem file used by xfs xfile

[Joonas Lahtinen]

Quoting Joonas Lahtinen (2024-01-10 17:20:24)
> 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.

To maybe write out my concern better:

Is there plan to enable huge pages by default in shmem?

If not I guess we should be pretty good with the way current code is, force
enabling them just might bring out some performance, so we might want to add
a warning for that.

If there is, then we'll probably want to in sync with those default changes
apply a similar call to block them on older HW.

Regards, Joonas

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