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

Baolin Wang
2026-04-11T07:22:15Z

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@redhat.com>
> ---
> 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):

@@ -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;