Re: [PATCH v3 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled

[Barry Song]

On Mon, Jun 23, 2025 at 8:28 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
> When invoking thp_vma_allowable_orders(), the TVA_ENFORCE_SYSFS flag is not
> specified, we will ignore the THP sysfs settings. Whilst it makes sense for the
> callers who do not specify this flag, it creates a odd and surprising situation
> where a sysadmin specifying 'never' for all THP sizes still observing THP pages
> being allocated and used on the system.
>
> The motivating case for this is MADV_COLLAPSE. The MADV_COLLAPSE will ignore
> the system-wide Anon THP sysfs settings, which means that even though we have
> disabled the Anon THP configuration, MADV_COLLAPSE will still attempt to collapse
> into a Anon THP. This violates the rule we have agreed upon: never means never.
>

Should we update the man page for madv_collapse ?
https://man7.org/linux/man-pages/man2/madvise.2.html

              MADV_COLLAPSE is independent of any sysfs (see sysfs(5))
              setting under /sys/kernel/mm/transparent_hugepage, both in
              terms of determining THP eligibility, and allocation
              semantics.  See Linux kernel source file
              Documentation/admin-guide/mm/transhuge.rst for more
              information.  MADV_COLLAPSE also ignores huge= tmpfs mount
              when operating on tmpfs files.  Allocation for the new
              hugepage may enter direct reclaim and/or compaction,
              regardless of VMA flags (though VM_NOHUGEPAGE is still
              respected).

So this effectively changes the uABI, right?

> Currently, besides MADV_COLLAPSE not setting TVA_ENFORCE_SYSFS, there is only
> one other instance where TVA_ENFORCE_SYSFS is not set, which is in the
> collapse_pte_mapped_thp() function, but I believe this is reasonable from its
> comments:
>
> "
> /*
>  * If we are here, we've succeeded in replacing all the native pages
>  * in the page cache with a single hugepage. If a mm were to fault-in
>  * this memory (mapped by a suitably aligned VMA), we'd get the hugepage
>  * and map it by a PMD, regardless of sysfs THP settings. As such, let's
>  * analogously elide sysfs THP settings here.
>  */
> if (!thp_vma_allowable_order(vma, vma->vm_flags, 0, PMD_ORDER))
> "
>
> Another rule for madvise, referring to David's suggestion: “allowing for
> collapsing in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".
>
> To address this issue, the current strategy should be:
>
> If no hugepage modes are enabled for the desired orders, nor can we enable them
> by inheriting from a 'global' enabled setting - then it must be the case that
> all desired orders either specify or inherit 'NEVER' - and we must abort.
>
> Meanwhile, we should fix the khugepaged selftest for MADV_COLLAPSE by enabling
> THP.

It’s a bit odd that the old test case expects collapsing to succeed
even when we’ve set it
to ‘never’.
Setting it to ‘always’ doesn’t seem to test anything as a counterpart.

I assume the goal is to test that setting it to ‘never’ prevents collapsing?

>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> ---

Thanks
Barry