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Received: from [96.67.55.146] (helo=fangorn.surriel.com) by shelob.surriel.com with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.97.1) (envelope-from ) id 1wtcb6-000000007kZ-2wGb; Mon, 10 Aug 2026 22:52:12 -0400 From: Rik van Riel To: linux-kernel@vger.kernel.org Cc: kernel-team@meta.com, Rik van Riel , Andrew Morton , David Hildenbrand , Jason Gunthorpe , John Hubbard , Peter Xu , linux-mm@kvack.org Subject: [RFC PATCH v3 7/8] mm/gup: walk multiple PTEs per follow_page_pte() call Date: Mon, 10 Aug 2026 22:51:56 -0400 Message-ID: <20260811025157.1632867-8-riel@surriel.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260811025157.1632867-1-riel@surriel.com> References: <20260811025157.1632867-1-riel@surriel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 1B777140004 X-Stat-Signature: so7x54ny4cyufts1dywkkz3k4bao1trz X-Rspam-User: X-HE-Tag: 1786417631-984219 X-HE-Meta: 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 b6dw1a4I 5v+QExMHlcIzKANX4wHYLceAmW6otFHMxWNvKpMnLx4+PGb/Pc9oOjWmzrkoF7CY31qdAGY2D9pDrezbXVRse7Kk2M8hGUl9WsMi/Dl2HTt2LJ7ve+n8l+H6zypOosvd1MNujjKt1laJCYyT1LKUFoIw2yZEIL9fp4nTG8tFSETUoGUyI0o/29TI8MHXFUL1vvDpuDmDDe4Zr/ulksEJCPyJcYfbjxeab4o+gO7X0lNsfKYhZfmJutmP90BAMtRDZseZHfRZbrsDTnmcKxhGFIeP6W+vvY9Ad/e6LvOOdzfiVtCC3SCrz5vncPU+vRB+eatYF1sJs63ySjeeFPGb7M76VHA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: follow_page_pte() looks at one PTE per call, so __get_user_pages() calls it once per page, restarting the pgd/p4d/pud/pmd descent and retaking the PTE lock each time. Walk every PTE from @address to the page-table/VMA/@end boundary in one call instead. Adjacent pages from different folios, or plain base pages with no folio relationship at all, are covered by the same call under one lock. A failure on the first PTE is returned as before. A failure after that ends the walk, and __get_user_pages() retrying the read gets the error. Measured with mm/gup_test.c, median get time over 16 iterations on a 256 MB MADV_HUGEPAGE region in a 4 CPU VM. Folio size was confirmed through the per-size anon_fault_alloc counters, 4100 folios for 64 kB and 128 for 2 MB: gup_test -L -m 256 -n 65536 -r 16 -t before after 4 kB base pages 2753 us 1178 us (2.3x) 64 kB mTHP 2946 us 1361 us (2.2x) 2 MB THP (control) 72 us 70 us Base pages and mTHP gain about the same amount, because what is saved is the page-table descent that no longer restarts per page, not anything folio-specific. 2 MB THP does not reach follow_page_pte(), so it stays flat. Suggested-by: David Hildenbrand Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Rik van Riel --- mm/gup.c | 62 ++++++++++++++++++++++++++++++++++++++------------------ 1 file changed, 42 insertions(+), 20 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index ea2bb379183e..c4233a7b8a48 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -916,38 +916,60 @@ static long follow_one_pte(struct vm_area_struct *vma, unsigned long address, return 0; } +/* + * Walk the PTEs from the start address to the end of this page table or VMA, + * whichever comes first, and commit every page found. + * + * A failure on the first PTE is returned to the caller. A failure after that + * is a short read; __get_user_pages() retrying the read will get the error. + */ static long follow_page_pte(struct vm_area_struct *vma, - unsigned long address, pmd_t *pmd, unsigned int flags, - struct page **pages) + unsigned long address, unsigned long end, pmd_t *pmd, + unsigned int flags, struct page **pages) { struct mm_struct *mm = vma->vm_mm; bool need_no_page_table = false; - struct page *page; + pte_t *ptep, *orig_ptep; + unsigned long walk_end; + unsigned long nr = 0; spinlock_t *ptl; - pte_t *ptep, pte; - long ret; + long ret = 0; - ptep = pte_offset_map_lock(mm, pmd, address, &ptl); + orig_ptep = ptep = pte_offset_map_lock(mm, pmd, address, &ptl); if (!ptep) return no_page_table(vma, flags, address); - pte = ptep_get(ptep); - - ret = follow_one_pte(vma, address, ptep, pte, flags, &page); - if (!ret && page) { - ret = follow_page_pte_commit(vma, address, page_folio(page), - page, pte, flags, pages); - if (!ret) - ret = 1; - } else if (!ret && pte_none(pte)) { + + walk_end = min(pmd_addr_end(address, end), vma->vm_end); + + for (; address < walk_end; address += PAGE_SIZE, ptep++) { + pte_t pte = ptep_get(ptep); + struct page *page; + + ret = follow_one_pte(vma, address, ptep, pte, flags, &page); + if (!ret && page) { + ret = follow_page_pte_commit(vma, address, + page_folio(page), page, + pte, flags, + pages ? pages + nr : NULL); + if (!ret) { + nr++; + continue; + } + } + /* * no_page_table() may look up the page cache, so it cannot run * under the PTE lock. */ - need_no_page_table = true; + if (!ret && pte_none(pte)) + need_no_page_table = true; + break; } - pte_unmap_unlock(ptep, ptl); + pte_unmap_unlock(orig_ptep, ptl); + if (nr) + return nr; if (need_no_page_table) return no_page_table(vma, flags, address); return ret; @@ -969,7 +991,7 @@ static long follow_pmd_mask(struct vm_area_struct *vma, if (!pmd_present(pmdval)) return no_page_table(vma, flags, address); if (likely(!pmd_leaf(pmdval))) - return follow_page_pte(vma, address, pmd, flags, pages); + return follow_page_pte(vma, address, end, pmd, flags, pages); if (pmd_protnone(pmdval) && !gup_can_follow_protnone(vma, flags)) return no_page_table(vma, flags, address); @@ -982,14 +1004,14 @@ static long follow_pmd_mask(struct vm_area_struct *vma, } if (unlikely(!pmd_leaf(pmdval))) { spin_unlock(ptl); - return follow_page_pte(vma, address, pmd, flags, pages); + return follow_page_pte(vma, address, end, pmd, flags, pages); } if (pmd_trans_huge(pmdval) && (flags & FOLL_SPLIT_PMD)) { spin_unlock(ptl); split_huge_pmd(vma, pmd, address); /* If pmd was left empty, stuff a page table in there quickly */ return pte_alloc(mm, pmd) ? -ENOMEM : - follow_page_pte(vma, address, pmd, flags, pages); + follow_page_pte(vma, address, end, pmd, flags, pages); } ret = follow_huge_pmd(vma, address, end, pmd, flags, pages); spin_unlock(ptl); -- 2.55.0