[PATCH 18/46] mm/khugepaged: check if PTE table is empty in try_collapse_pte_mapped_thp()

[sashiko-bot at kernel.org]

> 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.
> 
> 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()")
> Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org?part=18