The patch titled Subject: mm/ksm: convert break_ksm() to use walk_page_range_vma() has been added to the -mm mm-unstable branch. Its filename is mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days ------------------------------------------------------ From: David Hildenbrand <david@xxxxxxxxxx> Subject: mm/ksm: convert break_ksm() to use walk_page_range_vma() Date: Fri, 21 Oct 2022 12:11:40 +0200 FOLL_MIGRATION exists only for the purpose of break_ksm(), and actually, there is not even the need to wait for the migration to finish, we only want to know if we're dealing with a KSM page. Using follow_page() just to identify a KSM page overcomplicates GUP code. Let's use walk_page_range_vma() instead, because we don't actually care about the page itself, we only need to know a single property -- no need to even grab a reference. So, get rid of follow_page() usage such that we can get rid of FOLL_MIGRATION now and eventually be able to get rid of follow_page() in the future. In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test unmerge performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this results in a performance degradation of ~2% (old: ~5010 MiB/s, new: ~4900 MiB/s). I don't think we particularly care for now. Interestingly, the benchmark reduction is due to the single callback. Adding a second callback (e.g., pud_entry()) reduces the benchmark by another 100-200 MiB/s. Link: https://lkml.kernel.org/r/20221021101141.84170-9-david@xxxxxxxxxx Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx> Cc: John Hubbard <jhubbard@xxxxxxxxxx> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Cc: Peter Xu <peterx@xxxxxxxxxx> Cc: Shuah Khan <shuah@xxxxxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/ksm.c | 49 +++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 39 insertions(+), 10 deletions(-) --- a/mm/ksm.c~mm-ksm-convert-break_ksm-to-use-walk_page_range_vma +++ a/mm/ksm.c @@ -39,6 +39,7 @@ #include <linux/freezer.h> #include <linux/oom.h> #include <linux/numa.h> +#include <linux/pagewalk.h> #include <asm/tlbflush.h> #include "internal.h" @@ -419,6 +420,39 @@ static inline bool ksm_test_exit(struct return atomic_read(&mm->mm_users) == 0; } +static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next, + struct mm_walk *walk) +{ + struct page *page = NULL; + spinlock_t *ptl; + pte_t *pte; + int ret; + + if (pmd_leaf(*pmd) || !pmd_present(*pmd)) + return 0; + + pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl); + if (pte_present(*pte)) { + page = vm_normal_page(walk->vma, addr, *pte); + } else if (!pte_none(*pte)) { + swp_entry_t entry = pte_to_swp_entry(*pte); + + /* + * As KSM pages remain KSM pages until freed, no need to wait + * here for migration to end. + */ + if (is_migration_entry(entry)) + page = pfn_swap_entry_to_page(entry); + } + ret = page && PageKsm(page); + pte_unmap_unlock(pte, ptl); + return ret; +} + +static const struct mm_walk_ops break_ksm_ops = { + .pmd_entry = break_ksm_pmd_entry, +}; + /* * We use break_ksm to break COW on a ksm page by triggering unsharing, * such that the ksm page will get replaced by an exclusive anonymous page. @@ -434,21 +468,16 @@ static inline bool ksm_test_exit(struct */ static int break_ksm(struct vm_area_struct *vma, unsigned long addr) { - struct page *page; vm_fault_t ret = 0; do { - bool ksm_page = false; + int ksm_page; cond_resched(); - page = follow_page(vma, addr, - FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE); - if (IS_ERR_OR_NULL(page)) - break; - if (PageKsm(page)) - ksm_page = true; - put_page(page); - + ksm_page = walk_page_range_vma(vma, addr, addr + 1, + &break_ksm_ops, NULL); + if (WARN_ON_ONCE(ksm_page < 0)) + return ksm_page; if (!ksm_page) return 0; ret = handle_mm_fault(vma, addr, _ Patches currently in -mm which might be from david@xxxxxxxxxx are selftests-vm-anon_cow-test-cow-handling-of-anonymous-memory.patch selftests-vm-factor-out-pagemap_is_populated-into-vm_util.patch selftests-vm-anon_cow-thp-tests.patch selftests-vm-anon_cow-hugetlb-tests.patch selftests-vm-anon_cow-add-liburing-test-cases.patch mm-gup_test-start-stop-read-functionality-for-pin-longterm-test.patch mm-gup_test-start-stop-read-functionality-for-pin-longterm-test-fix.patch selftests-vm-anon_cow-add-r-o-longterm-tests-via-gup_test.patch selftests-vm-add-ksm-unmerge-tests.patch mm-pagewalk-dont-trigger-test_walk-in-walk_page_vma.patch selftests-vm-add-test-to-measure-madv_unmergeable-performance.patch mm-ksm-simplify-break_ksm-to-not-rely-on-vm_fault_write.patch mm-remove-vm_fault_write.patch mm-ksm-fix-ksm-cow-breaking-with-userfaultfd-wp-via-fault_flag_unshare.patch mm-pagewalk-add-walk_page_range_vma.patch mm-ksm-convert-break_ksm-to-use-walk_page_range_vma.patch mm-gup-remove-foll_migration.patch