Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Jun 25, 2025 at 04:52:03PM +0800, Baolin Wang wrote:
>
>
> On 2025/6/25 16:37, Lorenzo Stoakes wrote:
> > Yeah maybe the best way is to just have another tunable for this?
> >
> > /sys/kernel/mm/transparent_hugepage/disable_collapse perhaps?
> >
> > What do you think Hugh, Baolin?
>
> I think it's not necessary to find a way to disable madvise_collapse.
> Essentially, it's a conflict between the semantics of madvise_collapse and
> the '/sys/kernel/mm/transparent_hugepage/enabled' interface. We should reach
> a consensus on the semantics first:
>
> Semantic 1: madv_collapse() should ignore any THP system settings, meaning
> we need to update the 'never' semantics in
> '/sys/kernel/mm/transparent_hugepage/enabled', which would only disable page
> fault and khugepaged, not including madvise_collapse. If we agree on this,
> then the 'never' for per-sized mTHP would have the same semantics, i.e.,
> when I set 64K mTHP to 'always' and 2M mTHP to 'never', madvise_collapse
> would still allow the collapse of 2M THP. We should document this clearly in
> case users still want 64K mTHP from madvise_collapse.

Right yeah, I mean this is in effect how things are now. So the task is
documentation.

>
>
> Semantic 2: madv_collapse() needs to respect THP system settings, which is
> what my patch does. Never means never, and we would need to update the
> documentation of madv_collapse() to make it clearer.

Yes, and indeed this is the choice.

I think, as David said, it comes down to whether we have a legit use case that
truly relies on this.

> > (One side note on PMD-sized MADV_COLLAPSE - this is basically completely
> > useless for 64 KB page size arm64 systems where PMD's are 512 MB :)
> >
> > Thoughts Baolin?
>
> We should not collapse 512MB THP on 64K pagesize kernel. So seems
> madv_collapse() can not work on 64K pagesize kernel.

Well I don't think anything would prevent this now right? So MADV_COLLAPSE is
pretty problematic on 64K pagesize kernels in general.

Anyway that's maybe a problem for another time :)

> >
> > * If we drop the series, also consider how we might provide mTHP-compatible
> >    MADV_COLLAPSE.
>
> Yes. Agree. Will be another mess I guess :)

Sigh yes sadly. Let's try and make it as unmessy as we can I guess :>)