Re: [PATCH RFC v4 11/12] mm/gup: batch contiguous PTE-mapped large folios in follow_page_mask()

[David Hildenbrand (Arm)]

On 7/25/26 00:29, Rik van Riel wrote:
> follow_page_mask() returns one page per call for a PTE-mapped large folio,
> so __get_user_pages() re-walks the page tables for every page of an mTHP
> even though the folio maps a contiguous run. The huge PMD and PUD paths
> already return the whole mapping in one step.
> 
> Report the contiguous run for the PTE case too. follow_pte_batch() uses
> folio_pte_batch_flags() to count consecutive present PTEs that map
> consecutive pages of the same folio with a uniform write bit, bounded by
> the page table, @end, the VMA, and the folio itself.
> 
> Keep the per-PTE guarantees that follow_page_pte() makes for the head page.
> folio_pte_batch_flags() with FPB_RESPECT_WRITE stops the run at a change in
> the write bit, so the whole run matches the head.
> 
> A writable run is safe for any access: a writable anon page is exclusive,
> so gup_must_unshare() cannot fire, and FOLL_WRITE is satisfied.
> 
> A read-only run is batched only for a plain read, since FOLL_WRITE would
> need a COW fault per page and FOLL_PIN would need a per-page
> gup_must_unshare() check.
> 
> Measured with mm/gup_test.c (PIN_LONGTERM_BENCHMARK, the slow
> pin_user_pages() path) on a 256 MB MADV_HUGEPAGE anonymous region in a
> 4 CPU VM, median get time over 16 iterations. Each folio size was confirmed
> through the per-size anon_fault_alloc counters (4096 folios for 64 kB, 128
> for 2 MB):
> 
>   gup_test -L -m 256 -n 65536 -r 16 -t
>                         before      after
>   64 kB mTHP            3140 us      412 us   (7.6x)
>   2 MB THP (control)      78 us       76 us
>   4 kB base (control)   3010 us     3042 us
> 
> The PMD-mapped 2 MB THP already returns the whole mapping in one step, so
> it stays fast and unchanged. The 4 kB baseline shows the per-page walk cost
> that the 64 kB case paid before this change; only the PTE-mapped large
> folio case improves.
> 
> Assisted-by: Claude:claude-opus-4.8
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>

Why is this patch part of this patch set?

This makes perfect sense independently, no?

-- 
Cheers,

David