Re: [PATCH mm-unstable v19 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

[Lance Yang]

On Fri, Jun 05, 2026 at 10:14:13AM -0600, Nico Pache wrote:
>Pass an order to collapse_huge_page to support collapsing anon memory to
>arbitrary orders within a PMD. order indicates what mTHP size we are
>attempting to collapse to.
>
>For non-PMD collapse we must leave the anon VMA write locked until after
>we collapse the mTHP-- in the PMD case all the pages are isolated, but in
>the mTHP case this is not true, and we must keep the lock to prevent
>access/changes to the page tables. This can happen if the rmap walkers hit
>a pmd_none while the PMD entry is currently unavailable due to being
>temporarily removed during the collapse phase.
>
>To properly establish the page table hierarchy without violating any
>expectations from certain architectures (e.g. MIPS), we must make sure to
>have the PMD reinstalled before the PTEs, and hold both PTE/PMD locks
>before calling update_mmu_cache_range() (if they are distinct locks).
>
>Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
>---

Nothing else jumped out at me. Anything left can be sorted out later, as
David and Lorenzo said :)

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