Re: [PATCH v2] mm: shmem: ignore sysfs configs for shmem forced collapse

[Baolin Wang]

On 9/14/26 10:43 AM, Andrew Morton wrote:
> On Mon, 14 Sep 2026 10:12:41 +0800 Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
> > According to Documentation/mm/transhuge.rst, MADV_COLLAPSE is expected to
> > ignore any THP or mTHP interface settings.
> >
> > However, after commit 26c7d8413aaf ("mm: thp: support "THPeligible" semantics
> > for mTHP with anonymous shmem"), performing MADV_COLLAPSE on shmem will
> > depend on /sys/.../hugepages-2048kB/shmem_enabled being set to "inherit"
> > (although that is the default), which can cause MADV_COLLAPSE to fail
> > unexpectedly.
>
> Userspace-visible runtime regression?

Yes, it could cause a userspace-visible performance regression.

> > Fix this by returning the result of shmem_huge_global_enabled() directly
> > when MADV_COLLAPSE is requested, allowing PMD-order collapse while ignoring
> > shmem THP/mTHP settings.
> >
> > Fixes: 26c7d8413aaf ("mm: thp: support "THPeligible" semantics for mTHP with anonymous shmem")
>
> Should we backport?

I'm not sure, since the issue is triggered by a rather uncommon 
configuration. But given that the fix is simple, I think it's fine to CC 
stable.