mm/gup: batch PTE-mapped large folios in follow_page_mask()

follow_page_mask() reports the size of the page it found with a page_mask:
a bitmask of the enclosing naturally-aligned huge page. That form can only describe a power-of-two block aligned to its own size, so a PTE-mapped large folio (mTHP) is walked one page at a time even though it maps a contiguous run, while the PMD/PUD-mapped cases already return their whole mapping in one step.

Patch 1 replaces the page_mask output with nr_pages (a plain count) and adds an @end argument bounding the walk, with no functional change: the huge PMD/PUD paths report the same stride as before, just as a count.

Patch 2 uses the new interface to batch the PTE case: follow_pte_batch() counts 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, then follow_page_pte() hands the whole run back in one call.

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.

Both patches apply standalone against mm-unstable; no other series is required.

Rik van Riel (2):
mm/gup: pass an end address to follow_page_mask() and return a page
count
mm/gup: batch contiguous PTE-mapped large folios in follow_page_mask()

 mm/gup.c | 139 ++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 97 insertions(+), 42 deletions(-)

base-commit: 7368ccdeff50c27809d3a8276c85bae7e402c96c
---
Cc: Andrew Morton <[email protected]>
Cc: [email protected]
To: [email protected]
Cc: Dave Hansen <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: "Liam R. Howlett" <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Usama Arif <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: [email protected]

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