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

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