Re: [PATCH v3 06/10] mm: shmem: drop has_transparent_hugepage() usage

Lance Yang
2026-04-10T16:00:02Z

On Wed, Apr 08, 2026 at 04:23:01PM -0400, Luiz Capitulino wrote:
>Shmem uses has_transparent_hugepage() in the following ways:
>
>- shmem_parse_one() and shmem_parse_huge(): Check if THP is built-in and
> if the CPU supports PMD-sized pages
>
>- shmem_init(): Since the CONFIG_TRANSPARENT_HUGEPAGE guard is outside
> the code block calling has_transparent_hugepage(), the
> has_transparent_hugepage() call is exclusively checking if the CPU
> supports PMD-sized pages
>
>While it's necessary to check if CONFIG_TRANSPARENT_HUGEPAGE is enabled
>in all cases, shmem can determine mTHP size support at folio allocation
>time. Therefore, drop has_transparent_hugepage() usage while keeping the
>CONFIG_TRANSPARENT_HUGEPAGE checks.
>
>Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
>---
> mm/shmem.c | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
>
>diff --git a/mm/shmem.c b/mm/shmem.c
>index b40f3cd48961..6f8b20d77e07 100644
>--- a/mm/shmem.c
>+++ b/mm/shmem.c
>@@ -689,7 +689,7 @@ static int shmem_parse_huge(const char *str)
> else
> return -EINVAL;
>
>- if (!has_transparent_hugepage() &&
>+ if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> huge != SHMEM_HUGE_NEVER && huge != SHMEM_HUGE_DENY)
> return -EINVAL;
>
>@@ -4664,8 +4664,7 @@ static int shmem_parse_one(struct fs_context *fc, struct fs_parameter *param)
> case Opt_huge:
> ctx->huge = result.uint_32;
> if (ctx->huge != SHMEM_HUGE_NEVER &&
>- !(IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>- has_transparent_hugepage()))
>+ !IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> goto unsupported_parameter;
> ctx->seen |= SHMEM_SEEN_HUGE;
> break;
>@@ -5451,7 +5450,7 @@ void __init shmem_init(void)
> #endif
>
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>- if (has_transparent_hugepage() && shmem_huge > SHMEM_HUGE_DENY)
>+ if (shmem_huge > SHMEM_HUGE_DENY)
> SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge;
> else
> shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */
>--

Although this patch simply drops the early has_transparent_hugepage()
checks from the shmem parse/init paths, shmem_allowable_huge_orders()
still returns 0 when thp_disabled_by_hw() is set.

So on hardware without PMD THP support:

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)
{
...
if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
return 0;
...
}

1) the fault path still falls back to order-0 allocation
2) do_set_pmd() still falls back
3) khugepaged won't collapse it either

Nothing jumped out at me, thanks!

Reviewed-by: Lance Yang <lance.yang@linux.dev>