Re: [PATCH v5 5/9] mm/collapse: state what a collapse may do in the policy

[Lance Yang]

On Wed, Oct 14, 2026 at 06:11:38PM +0100, Kiryl Shutsemau (Meta) wrote:
>Tests scattered through the collapse path decide what a collapse is
>allowed to do by asking whether khugepaged started it.  Between them they
>settle:
>
> - which VMAs are eligible, and how hard to try for a folio;
> - how many empty, swapped-out or shared PTEs a window may contain, and
>   whether a sub-PMD window is held to a stricter rule than a PMD;
> - whether a range has to look used, and whether a MADV_FREE'd page is
>   left alone;
> - whether the PMD is mapped as part of the request, and whether dirty
>   pages are worth writing back and retrying.
>
>None of those is a fact about khugepaged.  Each is something the caller
>decided before asking, and the collapse code should not have to look up
>who called to find out.
>
>Add struct collapse_policy for the caller to fill: khugepaged from its
>own settings, MADV_COLLAPSE from the fact that a user asked explicitly.
>Every test becomes a read of a field, and cc->is_khugepaged goes, having
>no reader left.
>
>The PTE limits come as two sets, one for a PMD-sized window and one for
>anything smaller, so that the helpers pick a set for the order and read
>it.  khugepaged takes no swapped-out or shared PTE into a sub-PMD window,
>and empty PTEs only when the knob says all or nothing; the warning for a
>knob value in between moves to where khugepaged fills its policy.  The
>fields only one side reads say which: anon_ or file_.  David Hildenbrand
>asked for both.
>
>khugepaged fills the policy once per scan pass, MADV_COLLAPSE once per
>call.  That is the one change in behaviour.  The max_ptes_* limits and the
>defrag setting behind the allocation mask are sampled once per pass rather
>than on every table.  A table scanned early in a pass and one scanned late
>are then treated alike.
>
>collapse_file() also drops a NULL check on the collapse_control.  It has
>one call site, reached only from collapse_single_pmd(), which dereferences
>cc unconditionally, so the check was already dead.
>
>Assisted-by: LLM
>Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
>Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
>---

Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>