Re: [PATCH 0/5] mm/gup: batch contiguous pages in follow_page_mask()

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

Why was this un-RFC'd? The only review the RFC received was 'please don't send
unfiltered AI slop'? :)

On Fri, Jul 31, 2026 at 11:15:35PM -0400, Rik van Riel wrote:
> follow_page_mask() walks the page tables one page at a time, even when
> the caller asked for a whole run of contiguous pages. Every page of a
> large folio re-enters the pmd/pud/pte walk and re-takes the page table
> lock.
>
> This series changes follow_page_mask() to return a page count and a
> new @end argument bounding how many pages remain, instead of a single
> struct page, so a walker can hand back more than one page per call.
>
> Patch 1 converts the follow_page_mask()/follow_p4d_mask()/
> follow_pud_mask()/follow_pmd_mask()/follow_page_pte() call chain to
> return a long instead of a struct page pointer or ERR_PTR(), with no
> functional change: every path still handles exactly one page.
>
> Patch 2 is pure code motion, splitting the "commit to a resolved page"
> tail of follow_page_pte() into its own follow_page_pte_commit(), no
> functional change.
>
> Patch 3 adds gup_fill_pages(), a small helper that fills pages[] and
> flushes caches for a run of subpages, and converts the three existing
> per-page call sites to use it with nr == 1, no functional change.
>
> Patch 4 has follow_huge_pud()/follow_huge_pmd() report the huge page's
> real subpage count instead of a separate *page_mask output, and
> retires *page_mask and __get_user_pages()'s dead second
> try_grab_folio() call and subpage loop.
>
> It also defers gup_fill_pages() past the pud/pmd unlock, so a 1 GB
> PUD-mapped folio doesn't hold that lock for a full array fill and
> cache flush.
>
> Patch 5 adds follow_pte_batch() and has follow_page_pte() call it once
> per contiguous same-folio run instead of once per page, so a
> PTE-mapped mTHP no longer restarts the walk and re-takes the PTE lock
> per subpage.
>
> This is the only patch that changes the number of page table walks or
> lock acquisitions.
>
> Benchmarked with mm/gup_test.c (PIN_LONGTERM_BENCHMARK, pin_user_pages
> + FOLL_LONGTERM, 256 MB region, median of 16 runs, folio formation
> verified via the per-size anon_fault_alloc counter):
>
>                        before      after
>   4 kB base pages      2721 us     1198 us   (2.3x)
>   64 kB mTHP           2929 us      201 us  (14.6x)
>   2 MB THP               73 us       69 us   (flat)
>
> The 4 kB result comes entirely from patch 5 merging two separate
> try_grab_folio() calls and lock acquisitions into one; folio size and
> PTE batching play no part in it.
>
> 64 kB mTHP adds the walk-restart avoidance on top. 2 MB THP is
> unaffected, since follow_huge_pmd() already handled it in one call.
>
> Patch 4's lock-hold-time change is a scalability argument, not a
> measured one -- it is not visible in this single-threaded benchmark.
>
> Suggested-by: David Hildenbrand <david@kernel.org>
>
> Rik van Riel (5):
>   mm/gup: convert follow_page_mask() to return a long
>   mm/gup: split follow_page_pte_commit() out of follow_page_pte()
>   mm/gup: add gup_fill_pages() and use it
>   mm/gup: return a huge page's full count from follow_page_mask()
>   mm/gup: walk multiple PTEs per follow_page_pte() call
>
>  mm/gup.c | 532 +++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 322 insertions(+), 210 deletions(-)
>

No link to https://lore.kernel.org/all/20260730035350.1fc95dd8@fangorn/ or
change log to indicate that this is the un-RFC'd version of that (or indicating
why you un-RFC'd it)?

> base-commit: fc02acf6ac0c
> --
> 2.53.0-Meta

--
Cheers, Lorenzo