Re: [PATCH v1 1/1] mm/damon: support MADV_COLLAPSE via DAMOS_COLLAPSE scheme action

[SeongJae Park]

On Tue, 31 Mar 2026 13:50:21 +0300 Stepanov Anatoly <stepanov.anatoly@xxxxxxxxxx> wrote:

> On 3/31/2026 1:46 PM, Stepanov Anatoly wrote:
> > On 3/31/2026 4:31 AM, SeongJae Park wrote:
> >> Hello Asier,
> >>
> >> On Mon, 30 Mar 2026 14:57:58 +0000 <gutierrez.asier@xxxxxxxxxxxxxxxxxxx> wrote:
> >>
> >>> From: Asier Gutierrez <gutierrez.asier@xxxxxxxxxxxxxxxxxxx>
> >>>
> >>> This patch set introces a new action:  DAMOS_COLLAPSE.
> >>>
> >>> For DAMOS_HUGEPAGE and DAMOS_NOHUGEPAGE to work, khugepaged should be
> >>> working, since it relies on hugepage_madvise to add a new slot. This
> >>> slot should be picked up by khugepaged and eventually collapse (or
> >>> not, if we are using DAMOS_NOHUGEPAGE) the pages. If THP is not
> >>> enabled, khugepaged will not be working, and therefore no collapse
> >>> will happen.
> >>
> >> I should raised this in a previous version, sorry.  But, that is only a half of
> >> the picture.  That is, khugepaged is not the single THP allocator for
> >> MADV_HUGEPAGE.  IIUC, MADV_HUGEPAGE-applied region also allocates huge pages in
> >> page fault time.  According to the man page,
> >>
> >>     The kernel will regularly scan the areas marked as huge page  candidates
> >>     to replace  them with huge pages.  The kernel will also allocate huge pages
> >>     directly when the region is naturally aligned to the huge page size (see
> >>     posix_memalign(2)).
> >>
> > I think key difference between DAMOS_HUGEPAGE and DAMOS_COLLAPSE is the granularity.
> > 
> > In DAMOS_HUGEPAGE case, the granularity is always VMA, even if the hot region is narrow.
> > It's true for both page-fault based collapse and khugepaged collapse.
> *page-fault THP allocation, not collapse of course.

Good point.  I think this difference can also help answering why DAMOS_COLLAPSE
was making less huge pages in the test.

> 
> > 
> > With DAMOS_COLLAPSE we can cover cases, when there's large VMA, for example,
> > which contains some hot VA region inside, so we can collapse just that region, not the whole VMA.

This also makes sense to me.  Also I think this aligns to what we discussed for
MADV_COLLAPSE intro.


Thanks,
SJ

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