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

On Sun Aug 16, 2026 at 6:45 PM EDT, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <[email protected]>
>
> Yes, I know, this is a lot of changes. But I'm happy with the overall state
> of the patchset and the only reason I tag it as RFC is that it is tricky
> to get 57 patches upstream.
>
> I wanted to give a view of the end state first. I will suggest a possible
> way to split it below.
>
> I would appreciate any feedback.
>
> TL;DR
> =====
>
> This replaces khugepaged's anonymous collapse with an engine that
> can collapse sub-PMD ranges. It is built around migration entries and
> frozen folios instead of heavy locking and isolation, aiming for better
> scalability and less disruption to the workload being collapsed.
>
> Why
> ===
>
> mTHP collapse landed in khugepaged in 7.2 and I was glad to see it.  We
> at Meta run arm64 with 64K base pages, where a PMD is 512M: PMD-order THP
> is of limited use at that size, and mTHP is exactly what we want.
>
> It turned out not to help us.
>
> khugepaged only ever looks at PMD-aligned windows, and it is not an easy
> limitation to lift.
>
> Fixing the alignment is a one-line change, but what it feeds assumes the
> PMD everywhere that matters: collapse_huge_page() clears and flushes the
> whole PMD whatever order it is collapsing, installs a PMD leaf because
> that is the only thing it can produce, and keeps everyone out with
> mmap_write_lock, anon_vma_lock_write() and an IPI broadcast while it
> does.
>
> Which is why hugepage_vma_revalidate() demands that the VMA span the
> whole PMD even for an mTHP order -- "we'd need to lock all VMAs in the
> PMD range to support this", as the comment there puts it.  A PMD-granular
> operation is only safe when one VMA owns the PMD, and that is exactly the
> restriction in the way.  The alignment is the symptom; the PMD is the
> design.
>
> So both roots have to go.

I agree that khugepaged is designed for PMD-aligned collapse and this is
a limitation we want to get rid of. It is great you are looking at them.

>
> Design
> ======
>
> The old mechanism holds the address space still because it has nothing
> else stopping the sources from moving under the copy.  The new engine
> makes the sources themselves inert instead, with the two barriers
> migration already uses, raised in that order:
>
>   1. migration entries replace the source PTEs.  Faults and GUP-slow
>      now wait on the source folio's lock, which is taken before the
>      first entry becomes visible.
>   2. the source folio's refcount is frozen to its expected value.
>      GUP-fast, pfn walkers, reclaim, compaction and memory-failure all
>      fail folio_try_get() and back off.
>
> Between the two, nothing can reach a source, so the copy runs with no
> lock held at all -- and the address space is left alone while it does.
>
> What that removes from every collapse path:
>
>   mmap_write_lock              -> mmap_read
>   anon_vma_lock_write()        -> nothing: an rmap walk needs the folio
>                                   locked, and the engine holds that lock
>                                   from freeze to putback
>   tlb_remove_table_sync_one()  -> nothing: one ranged flush per round
>   LRU isolation                -> nothing: sources are inert in place

I remember we were discussing using migration entry and the issue with
mmap_write_lock() in the context of in-place THP promotion and the
conclusion was that because MADV_DONTNEED (maybe MADV_REMOVE or
MADV_PAGEOUT) works on page table and does not change VMAs,
mmap_write_lock() is needed to prevent things being changed under
khugepaged. Anything different in normal khugepaged collapse process, so
that it is OK to use mmap_read_lock? Let me know if I misremember it.

Thanks.

-- 
Best Regards,
Yan, Zi