GUP-fast grabs a PTE-mapped large folio one page at a time. Every PTE costs a try_grab_folio_fast() (a refcount cmpxchg, plus the pincount atomic, a full barrier and a node stat update for FOLL_PIN), a gup_fast_folio_allowed() and a folio_set_referenced(), so a 64 kB mTHP pays sixteen of each and a PTE-mapped 2 MB THP pays 512. The users are the hot ones: iov_iter_extract_user_pages() pins every O_DIRECT buffer through pin_user_pages_fast(), io_uring registers its buffers through it, and so does RDMA memory registration. With mTHP those buffers are PTE-mapped large folios. After the head page has been grabbed and verified as before, extend the run to the following PTEs that map the next pages of the same folio with the same protection bits, take the remaining references for the run with try_grab_folio(), and fill pages[] from the run. The PTEs of the run are read while the reference on the folio is held, so the folio can neither be collapsed, freed nor split underneath the scan. A read-only run of an anonymous folio stops at the first page that gup_must_unshare() rejects, so the per-page exclusivity guarantee for FOLL_PIN is kept. folio_pte_batch_flags() does the scan. It reads the entries with ptep_get(), which is not safe without the page table lock on CONFIG_GUP_GET_PXX_LOW_HIGH (x86 PAE, mips32, sh) or with arm64 contpte, and GUP-fast reads every PTE with ptep_get_lockless() for that reason. Add FPB_LOCKLESS to make the helper do the same; it is inlined per call site, so the other callers do not change. The order-0 path keeps its instruction sequence. The folio flags are tested before folio_set_referenced() on purpose: a load of the flags right after that locked instruction cost about 25% on order-0 pages in the gup_test benchmark below, and reading them first brings that path back to the baseline. gup_test ioctl over a 256 MB anonymous region, FOLL_WRITE, 65536 pages per call, median of 15 runs, two baseline boots / one patched boot, on an x86-64 VM: get_user_pages_fast() pin_user_pages_fast() before after before after 4 kB pages 849/789 us 783 us 1174/1138 us 1094 us 64 kB mTHP 812/811 us 126 us 1215/1113 us 188 us 1 MB mTHP 799/774 us 59 us 1185/1077 us 64 us 2 MB THP, PTE-mapped 803/786 us 57 us 1159/1120 us 58 us 2 MB THP, PMD-mapped 42/40 us 40 us 46/41 us 42 us fio O_DIRECT randread of a null_blk device (CPU bound) with the I/O buffer in 64 kB mTHP, median of 15 five-second runs: bs=1M psync 126.2/125.6 GB/s -> 220.6 GB/s bs=1M io_uring, iodepth 16 71.2/71.6 GB/s -> 112.1 GB/s bs=64k io_uring, iodepth 16 50.2/50.7 GB/s -> 60.1 GB/s IORING_REGISTER_BUFFERS of 1 GB, median of 15: 64 kB mTHP 6179/5998 us -> 1599 us 1 MB mTHP 5790/5801 us -> 893 us 4 kB pages 7173/6741 us -> 6763 us Reviewed-by: David Hildenbrand (Arm) Reviewed-by: Rik van Riel Co-developed-by: Yuan-Hao Hsu Signed-off-by: Yeoreum Yun --- mm/gup.c | 65 +++++++++++++++++++++++++++++++++++++++------------------------- 1 file changed, 40 insertions(+), 25 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index 8ef41353afe8..b0a0019de80a 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2936,7 +2936,8 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, unsigned long end, unsigned int flags, struct page **pages) { - unsigned long nr_pages = 0; + unsigned long nr_pages = 0, nr_batch; + struct folio *folio = NULL; pte_t *ptep, *ptem; ptem = ptep = pte_offset_map(&pmd, addr); @@ -2945,7 +2946,9 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, do { pte_t pte = ptep_get_lockless(ptep); struct page *page; - struct folio *folio; + unsigned long i; + + nr_batch = 1; /* * Always fallback to ordinary GUP on PROT_NONE-mapped pages: @@ -2955,13 +2958,13 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, * permissions. */ if (pte_protnone(pte)) - goto pte_unmap; + goto out; if (!pte_access_permitted(pte, flags & FOLL_WRITE)) - goto pte_unmap; + goto out; if (pte_special(pte)) - goto pte_unmap; + goto out; /* If it's not marked as special it must have a valid memmap. */ VM_WARN_ON_ONCE(!pfn_valid(pte_pfn(pte))); @@ -2969,7 +2972,7 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, folio = try_grab_folio_fast(page, 1, flags); if (!folio) - goto pte_unmap; + goto out; /* * Recheck the PTE after grabbing the folio to detect concurrent @@ -2980,19 +2983,25 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, * would not detect that we are walking a detached page table. */ if (unlikely(pmd_val(pmd) != pmd_val(pmdp_get_lockless(pmdp))) || - unlikely(pte_val(pte) != pte_val(ptep_get_lockless(ptep)))) { - gup_put_folio(folio, 1, flags); - goto pte_unmap; - } + unlikely(pte_val(pte) != pte_val(ptep_get_lockless(ptep)))) + goto out; - if (!gup_fast_folio_allowed(folio, flags)) { - gup_put_folio(folio, 1, flags); - goto pte_unmap; - } + if (!gup_fast_folio_allowed(folio, flags)) + goto out; - if (!pte_write(pte) && gup_must_unshare(NULL, flags, folio, page)) { - gup_put_folio(folio, 1, flags); - goto pte_unmap; + if (folio_test_large(folio)) + /* + * The reference acquired above keeps the folio stable + * for the following checks. + */ + nr_batch = folio_pte_batch_flags(folio, NULL, ptep, &pte, + (end - addr) >> PAGE_SHIFT, + FPB_LOCKLESS | FPB_RESPECT_WRITE); + + if (!pte_write(pte)) { + nr_batch = gup_pages_not_requiring_unshare(NULL, flags, folio, page, nr_batch); + if (!nr_batch) + goto out; } /* @@ -3001,16 +3010,22 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, * see Documentation/core-api/pin_user_pages.rst for * details. */ - if ((flags & FOLL_PIN) && arch_make_folio_accessible(folio)) { - gup_put_folio(folio, 1, flags); - goto pte_unmap; - } + if ((flags & FOLL_PIN) && arch_make_folio_accessible(folio)) + goto out; + + if (nr_batch > 1 && try_grab_folio(folio, nr_batch - 1, flags)) + nr_batch = 1; + folio_set_referenced(folio); - pages[nr_pages] = page; - nr_pages++; - } while (ptep++, addr += PAGE_SIZE, addr != end); + for (i = 0; i < nr_batch; i++) + pages[nr_pages + i] = page + i; + nr_pages += nr_batch; + folio = NULL; + } while (ptep += nr_batch, addr += nr_batch * PAGE_SIZE, addr != end); -pte_unmap: +out: + if (folio) + gup_put_folio(folio, 1, flags); pte_unmap(ptem); return nr_pages; } -- 2.43.0