Re: [PATCH v2 1/2] mm: free unused page table allocated by do_set_pmd()

[Baolin Wang]

On 11/6/26 8:36 PM, David Hildenbrand (Arm) wrote:
> do_set_pmd() can allocate vmf->prealloc_pte on architectures that need a
> deposited page table, such as PowerPC with the hash MMU. If the PMD is
> no longer empty when it takes the PMD lock, it returns VM_FAULT_FALLBACK
> without consuming that table.
>
> Since collapse_pte_mapped_thp() uses mmap_read_lock(), a concurrent
> userfaultfd operation can install a PTE table before taking the folio
> lock. If that races with set_huge_pmd(), the unused preallocated table
> is lost when its local vm_fault goes out of scope.
>
> Track whether do_set_pmd() allocated the table and free it after dropping
> the PMD lock if it was not consumed. Preserve any preallocated table
> supplied by the caller for PTE fallback. Successful PMD installation
> consumes the table and clears the pointer.
>
> Returning without a preallocated table matches the existing behavior on
> architectures that do not need a deposited table. All callers
> already handle that case.
>
> Fixes: 1043173eb5eb ("mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()")
> Cc: stable@xxxxxxxxxxxxxxx
> Reviewed-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>