Re: [PATCH 7.2 v2 05/12] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check in hugepage_pmd_enabled()

Zi Yan
2026-04-13T20:42:54Z

On 13 Apr 2026, at 16:33, Matthew Wilcox wrote:

> On Mon, Apr 13, 2026 at 03:20:23PM -0400, Zi Yan wrote:
>> After READ_ONLY_THP_FOR_FS Kconfig is removed, this check becomes dead
>> code.
>>
>> This changes hugepage_pmd_enabled() semantics. Previously, with
>> READ_ONLY_THP_FOR_FS enabled, hugepage_pmd_enabled() returned true whenever
>> /sys/kernel/mm/transparent_hugepage/enabled was set to "always" or
>> "madvise".
>>
>> After this change, hugepage_pmd_enabled() is governed only by the anon and
>> shmem PMD THP controls. As a result, khugepaged collapse for file-backed
>> folios no longer runs unconditionally under the top-level THP setting, and
>> now depends on the anon/shmem PMD configuration.
>
> This seems like it'll turn off khugepaged too easily. I would have
> thought we'd want:
>
> - if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
> - hugepage_global_enabled())
> + if (hugepage_global_enabled())
> return true;

I thought about this, but it means khugepaged is turned on regardless of
anon and shmem configs. I tend to think the original code was a bug,
since enabling CONFIG_READ_ONLY_THP_FOR_FS would enable khugepaged all
the time.

>
> ... or maybe this whole thing could be simplified?

Alternatives could be:
1. to add a file-backed khhugepaged config, but another sysfs?
2. to replace hugepage_pmd_enabled() with hugepage_global_enabled()
and let thp_vma_allowable_order() in collapse_scan_mm_slot()
skip not qualified VMAs, but that would waste extra CPU cycles
for scanning. Maybe not too much waste.

Best Regards,
Yan, Zi