Re: [PATCH v3 18/47] mm/khugepaged: check if PTE table is empty in try_collapse_pte_mapped_thp()

[Lance Yang]

On Sat, Oct 31, 2026 at 10:39:47AM +0100, David Hildenbrand (Arm) wrote:
>The existing code is rather horrible and already resulted in bugs in the
>past: we special-case UFFD inserting anonymous folios by grabbing the PMD
>lock early, but ignore any other markers (guard, poison through uffd) that
>could have been inserted into the page table in the meantime, effectively
>dropping them.
>
>Let's just unify and simplify the locking: try to grab the PMD table
>lock and if that fails, flush the TLB, drop the PTE lock, take PT locks in
>the right order and check if the page table is still empty.

Yep ;)

Taking the PMD lock up front would make the locking easier to follow, though
we'd hold it across all the PTE zapping ;)

>We still perform the heavy lifting (zapping the folio ptes) under PTE table
>lock only.
>
>Include <linux/hugetlb.h> for now, as x86's flush_tlb_range() macro
>depends on hugetlb helpers. This dependency needs a separate cleanup.
>
>Both issues this fixes are rather corner cases. Let's not CC stable for
>now.
>
>Fixes: f807123d578d ("mm: allow guard regions in file-backed and read-only mappings")
>Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()")
>Tested-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
>Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
>Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
>Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>---

LGTM!

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