mm/collapse: state what a collapse may do in the policy

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.

Link: https://lore.kernel.org/20260928100630.21870-6-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Assisted-by: LLM
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>

0 files changed, 0 insertions, 0 deletions