Re: [RFC PATCH 00/57] mm/collapse: rebuild collapse on migration primitives

[Kiryl Shutsemau <kirill@shutemov.name>]

On Tue, Aug 18, 2026 at 04:12:17PM +0200, David Hildenbrand (Arm) wrote:
> I think we all agree that there is a lot of room for improvement, but the big
> question is:
> 
> (a) When does it stop being a cleanup and is a new feature in disguise that
>     makes the code more complicated and even harder to maintain.
> 
> (b) Can it just naturally be made looking like a cleanup.
> 
> Ideally, we'd get b), in small, nice-to-review chunks that incrementally improve
> the code without inflating it heavily or moving everything around.

It is not a cleanup and I would rather not sell it as one.  It replaces a
mechanism, so judged as (b) it fails by construction.

I believe the end result is much cleaner. But I might be biased. :)

> The current locking is nasty, so anything that moves us one step closer into
> something that is not only simpler but also more scalable is nice. I am a bit
> concerned with the churn in the series as is.
> 
> After this series, mm/collapse.c itself is way larger than just mm/khugepaged.c
> originally, which raises some eyebrows.

Line count is a poor proxy for simplicity or scalability.  What the
engine changes is the serialization model, and that is the part collapse
needs changed: the PMD granularity and the exclusion both come out of
the locking.

Incremental does not reach it, though.  The old mechanism is correct
because it holds mmap_write_lock, the anon_vma write lock and a reference
from the LRU; the engine is correct because the sources are frozen behind
migration entries.  There is no halfway state that is correct under both,
so the switch lands as one patch.

What can be incremental is everything around it: the engine goes in beside
the old mechanism, patch 25 points the anon path at it, and 28 removes what
it replaces.  Until 28 both are in the tree with only one of them
reachable, so the switch can be reverted on its own.

> We should also be aware that people are proposing file/shmem mTHP collapse, so
> ideally what we refactor would naturally unify some of these code paths.
> 
> I am wondering whether shmem mTHP collapse should come first. (I'm hoping that
> shmem mTHP collapse can unify some of the anon+file collapse code in a nice way,
> to similarly just look like a cleanup while enabling a new scenario.

mTHP collapse as it stands has limited usability: PMD-aligned windows only,
and one VMA has to own the PMD.  Bolting file collapse onto the same
structure adds to the debt instead of paying it down.

It would fit the new design.  The frame -- scan, candidate selection, the
round and its passes -- has nothing anon-specific in it; what is
anon-specific sits in the freeze (folio_test_anon(), PageAnonExclusive())
and the unshare in the fault-in pass.  A file source would bring its own
check, freeze, copy and install.

I am not sure it should, though.

Do we want to find file collapse candidates by walking the virtual
address space at all?

collapse_file() already works on the mapping -- it builds the folio in
the page cache and then repairs every mapping through
retract_page_tables() -- so the VMA walk only picks which inode range to
try, and it reaches only what a registered mm maps right now.  Large
folios buy more than TLB reach: fewer page cache entries, cheaper
writeback, natural locking batch, etc.  Those apply whether the file is
mapped or not, and going at the inode directly would reach them.

> Agreed, I think we really should unify+cleanup the existing code first before
> doing more drastic changes.
> 
> Having a series that throws all of khugepaged.c into a mixer and pours something
> new into collapse.c is ... concerning :)

The moving around is patches 29-35 and the tracing after them.  None of it
is needed for the engine: 1-28 add it, switch the anon path over and delete
the old mechanism, without moving anything else out of khugepaged.c.  If
the churn is the problem, v2 can stop there and the moves can come later as
their own series.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov