Re: [PATCH v12 mm-new 15/15] Documentation: mm: update the admin guide for mTHP collapse

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

On Wed, Oct 22, 2025 at 10:22:08PM +0200, David Hildenbrand wrote:
> On 22.10.25 21:52, Christoph Lameter (Ampere) wrote:
> > On Wed, 22 Oct 2025, Nico Pache wrote:
> >
> > > Currently, madvise_collapse only supports collapsing to PMD-sized THPs +
> > > and does not attempt mTHP collapses. +
> >
> > madvise collapse is frequently used as far as I can tell from the THP
> > loads being tested. Could we support madvise collapse for mTHP?
>
> The big question is still how user space can communicate the desired order,
> and how we can not break existing users.

Yes, and let's go one step at a time, this series still needs careful scrutiny
and we need to ensure the _fundamentals_ are in place for khugepaged before we
get into MADV_COLLAPSE :)

>
> So I guess there will definitely be some support to trigger collapse to mTHP
> in the future, the big question is through which interface. So it will
> happen after this series.

Yes.

>
> Maybe through process_madvise() where we have an additional parameter, I
> think that was what people discussed in the past.

I wouldn't absolutely love us doing that, given it is a general parameter so
would seem applicable to any madvise() option and could lead to confusion, also
process_madvise() was originally for cross-process madvise vector operations.

I expanded this to make it applicable to the current process (and introduced
PIDFD_SELF to make that more sane), and SJ has optimised it across vector
operations (thanks SJ! :), but in general - it seems very weird to have
madvise() provide an operation that process_madvise() providse another version
of that has an extra parameter.

As usual we've painted ourselves into a corner with an API... :)

Perhaps we'll to accept the process_madvise() compromise and add
MADV_COLLAPSE_MHTP that only works with it or something.

Of course adding a new syscall isn't impossible... madvise2() not very appealing
however...

TL;DR I guess we'll deal with that when we come to it :)

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo