Re: [PATCH v3 09/10] mm: thp: always enable mTHP support

[Luiz Capitulino]

On 2026-04-09 16:10, Zi Yan wrote:
> On 9 Apr 2026, at 16:07, Luiz Capitulino wrote:
>
> > On 2026-04-09 11:55, Zi Yan wrote:
> > > On 8 Apr 2026, at 16:23, Luiz Capitulino wrote:
> > >
> > > > If PMD-sized pages are not supported on an architecture (ie. the
> > > > arch implements arch_has_pmd_leaves() and it returns false) then the
> > > > current code disables all THP, including mTHP.
> > > >
> > > > This commit fixes this by allowing mTHP to be always enabled for all
> > > > archs. When PMD-sized pages are not supported, its sysfs entry won't be
> > > > created and their mapping will be disallowed at page-fault time.
> > > >
> > > > Similarly, this commit implements the following changes for shmem:
> > > >
> > > >    - In shmem_allowable_huge_orders(): drop the pgtable_has_pmd_leaves()
> > > >      check so that mTHP sizes are considered
> > > >    - In shmem_alloc_and_add_folio(): don't consider PMD and PUD orders
> > > >      when PMD-sized pages are not supported by the CPU
> > > >
> > > > Signed-off-by: Luiz Capitulino <luizcap@xxxxxxxxxx>
> > > > ---
> > > >    mm/huge_memory.c | 13 ++++++++-----
> > > >    mm/shmem.c       |  4 +++-
> > > >    2 files changed, 11 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > > index 86e489c0a150..6de3d8ebc35c 100644
> > > > --- a/mm/huge_memory.c
> > > > +++ b/mm/huge_memory.c
> > > > @@ -118,6 +118,9 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> > > >    	else
> > > >    		supported_orders = THP_ORDERS_ALL_FILE_DEFAULT;
> > > >
> > > > +	if (!pgtable_has_pmd_leaves())
> > > > +		supported_orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
> > >
> > > Why is BIT(PUD_ORDER) also removed? I thought PMD THP support and PUD THP support
> > > are separate. Here the code implies PUD THP relies on PMD THP. Is that the case?
> >
> > This was a suggestion from David to an earlier version:
> >
> > https://lore.kernel.org/linux-mm/dac20466-adac-4e47-8f50-87f4774fd57b@xxxxxxxxxx/
> >
> > My understanding was that if an arch doesn't support PMD pages then it
> > probably doesn't support PUD pages either.
>
> Got it. Can you add a comment on “No PMD leaves -> No PUD leaves” for
> pgtable_has_pmd_leaves(), so that no one would ask the same question again?

Sure, I can do that.

> Thanks.
>
> Best Regards,
> Yan, Zi