Re: disable large folios for shmem file used by xfs xfile

[Daniel Gomez <dagmcr@gmail.com>]

On Wed, Jan 10, 2024 at 4:35 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> 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 ;-)

I have sent large folios support for shmem for the write and fallocate
path some releases ago. The main problem I was facing was a current
upstream problem with huge pages when seeking holes/data (fstests
generic/285 and generic/436). The strategy suggested was to use large
folios in an opportunistic way based on the file size. This hit the
same problem we currently have with huge pages and I considered that a
regression. We have made some progress to fix seeking in huge pages
upstream but is not yet finished. I can send the patches tomorrow for
further discussion.

>