disable large folios for shmem file used by xfs xfile

[Matthew Wilcox]

On Wed, Jan 10, 2024 at 05:28:22PM +0200, Joonas Lahtinen wrote:
> Quoting Joonas Lahtinen (2024-01-10 17:20:24)
> > 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?

Not in the next kernel release, but eventually the plan is to allow
arbitrary order folios to be used in shmem.  So you could ask it to create
a 256kB folio for you, if that's the right size to manage memory in.

How shmem and its various users go about choosing the right size is not
quite clear to me yet.  Perhaps somebody else will do it before I get
to it; I have a lot of different sub-projects to work on at the moment,
and shmem isn't blocking any of them.  And I have a sneaking suspicion
that more work is needed in the swap code to deal with arbitrary order
folios, so that's another reason for me to delay looking at this ;-)