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

[Luiz Capitulino]

On 2026-04-11 03:22, Baolin Wang wrote:
> On 4/9/26 4:23 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 | 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));
> > +
> >       orders &= supported_orders;
> >       if (!orders)
> >           return 0;
> > @@ -125,7 +128,7 @@ 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. */
> > @@ -787,7 +790,7 @@ static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
> >        * disable all other sizes. powerpc's PMD_ORDER isn't a compile-time
> >        * constant so we have to do this here.
> >        */
> > -    if (!anon_orders_configured)
> > +    if (!anon_orders_configured && pgtable_has_pmd_leaves())
> >           huge_anon_orders_inherit = BIT(PMD_ORDER);
> >       *hugepage_kobj = kobject_create_and_add("transparent_hugepage", mm_kobj);
> > @@ -809,6 +812,9 @@ 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));
> > +
> >       order = highest_order(orders);
> >       while (orders) {
> >           thpsize = thpsize_create(order, *hugepage_kobj);
> > @@ -908,9 +914,6 @@ static int __init hugepage_init(void)
> >       int err;
> >       struct kobject *hugepage_kobj;
> > -    if (!pgtable_has_pmd_leaves())
> > -        return -EINVAL;
> > -
> >       /*
> >        * hugepages can't be allocated by the buddy allocator
> >        */
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 613393eae5a9..b49a30475cb0 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1839,7 +1839,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
> >       vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
> >       unsigned int global_orders;
> > -    if (!pgtable_has_pmd_leaves() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
> > +    if (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force))
> >           return 0;
> >       global_orders = shmem_huge_global_enabled(inode, index, write_end,
> > @@ -1947,6 +1947,8 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
> >       if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> >           orders = 0;
> > +    else if (!pgtable_has_pmd_leaves())
> > +        orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
> Sorry, I still don't like the changes here, because shmem_allowable_huge_orders() is meant to determine which large orders are allowed. Something like below (untested):

Sure, I can do something like this.

> @@ -1834,6 +1834,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>                                  struct vm_area_struct *vma, pgoff_t index,
>                                  loff_t write_end, bool shmem_huge_force)
>   {
> +       unsigned int filter_orders = pgtable_has_pmd_leaves() ? (BIT(PMD_ORDER) | BIT(PUD_ORDER)) : 0;
>          unsigned long mask = READ_ONCE(huge_shmem_orders_always);
>          unsigned long within_size_orders = READ_ONCE(huge_shmem_orders_within_size);
>          vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
> @@ -1846,7 +1847,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>                                                    shmem_huge_force, vma, vm_flags);
>          /* Tmpfs huge pages allocation */
>          if (!vma || !vma_is_anon_shmem(vma))
> -               return global_orders;
> +               return global_orders & ~filter_orders;
>
>          /*
>           * Following the 'deny' semantics of the top level, force the huge
> @@ -1871,6 +1872,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>          if (global_orders > 0)
>                  mask |= READ_ONCE(huge_shmem_orders_inherit);
>
> +       mask &= ~filter_orders;
>          return THP_ORDERS_ALL_FILE_DEFAULT & mask;
>   }
>
>
> Additionally, we also need a pgtable_has_pmd_leaves() check before setting huge_shmem_orders_inherit as well.
>
> @@ -5428,7 +5430,7 @@ void __init shmem_init(void)
>           * Default to setting PMD-sized THP to inherit the global setting and
>           * disable all other multi-size THPs.
>           */
> -       if (!shmem_orders_configured)
> +       if (!shmem_orders_configured && pgtable_has_pmd_leaves())
>                  huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
>   #endif
>          return;