Re: [PATCH v2 10/11] mm: thp: always enable mTHP support

[Baolin Wang]

On 2/10/26 9:28 PM, Luiz Capitulino wrote:
> On 2026-02-10 04:56, Baolin Wang wrote:
> > On 2/10/26 6:14 AM, 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 | 11 +++++++----
> > >   mm/shmem.c       |  4 +++-
> > >   2 files changed, 10 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 1e5ea2e47f79..882331592928 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -115,6 +115,9 @@ 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));
> > > +
> > >       orders &= supported_orders;
> > >       if (!orders)
> > >           return 0;
> > > @@ -122,7 +125,7 @@ unsigned long __thp_vma_allowable_orders(struct 
> > > vm_area_struct *vma,
> > >       if (!vma->vm_mm)        /* vdso */
> > >           return 0;
> > > -    if (!pgtable_has_pmd_leaves() || vma_thp_disabled(vma, vm_flags, 
> > > forced_collapse))
> > > +    if (vma_thp_disabled(vma, vm_flags, forced_collapse))
> > >           return 0;
> > >       /* khugepaged doesn't collapse DAX vma, but page fault is fine. */
> > > @@ -806,6 +809,9 @@ static int __init hugepage_init_sysfs(struct 
> > > kobject **hugepage_kobj)
> > >       }
> > >       orders = THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_FILE_DEFAULT;
> > > +    if (!pgtable_has_pmd_leaves())
> > > +        orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
> >
> > I think you should also handle the 'huge_anon_orders_inherit' setting 
> > in this function if pgtable_has_pmd_leaves() returns false. Shmem as 
> > well.
> >
> > if (!anon_orders_configured)
> >      huge_anon_orders_inherit = BIT(PMD_ORDER);
>
> Good catch. So, would you agree that should set it to BIT(PMD_ORDER - 1)
> in this case?

From the documentation:
"
By default, PMD-sized hugepages have enabled="inherit" and all other
hugepage sizes have enabled="never".
"

So if pgtable_has_pmd_leaves() returns false, IMO, we should just skip 
setting the PMD-sized order for huge_anon_orders_inherit. What I mean is:

if (!anon_orders_configured && pgtable_has_pmd_leaves())
	huge_anon_orders_inherit = BIT(PMD_ORDER);