Re: [RFC PATCH v3 8/8] mm/gup: batch contiguous same-folio PTEs into one refcount grab

[Yuan-Hao Hsu]

On Mon, 10 Aug 2026 22:51:57 -0400, Rik van Riel wrote:
> Add follow_pte_batch() and give follow_page_pte_commit() a run length.
> Each run starts with a full per-PTE resolve, then follow_pte_batch()
> finds how many more PTEs extend it with a pte_same() scan rather than a
> re-derivation of each page, and the run is committed with one refcount
> grab. Runs stop at folio boundaries, so plain base pages are unaffected.

One side effect worth a sentence in the changelog: with FOLL_TOUCH,
folio_mark_accessed() now runs once per run instead of once per page.
On the classic LRU the first call only sets PG_referenced and the
second one activates the folio, so the old per-page loop activated a
PTE-mapped large folio from its second page on, and a run now leaves
it referenced but inactive.

Measured with one get_user_pages() call over 2 MB of freshly faulted
64 kB folios (kprobe on folio_mark_accessed(), flags from
/proc/kpageflags):

                       before            after
  folio_mark_accessed  512 per call      32 per call
  after 1st call       32 folios active  32 referenced, 0 active
  after 2nd call       32 active         32 active

That matches what the other two paths already do (follow_huge_pmd()
only calls touch_pmd(), GUP-fast only calls folio_set_referenced()),
so it looks like the right outcome.  It is still a visible change in
aging for anything that reaches these folios through get_user_pages()
rather than the fast path.

Yuan-Hao Hsu