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([2804:30c:274c:de00:6aad:5873:dac7:9b5c]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-1244ad740c5sm3537705c88.8.2026.01.16.09.22.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 16 Jan 2026 09:22:47 -0800 (PST) From: Pedro Demarchi Gomes To: stable@vger.kernel.org Cc: Zhi.Yang@windriver.com, Pedro Demarchi Gomes , David Hildenbrand , craftfever , Chengming Zhou , xu xin , Andrew Morton Subject: [PATCH 5.15.y 2/2] ksm: use range-walk function to jump over holes in scan_get_next_rmap_item Date: Fri, 16 Jan 2026 14:19:14 -0300 Message-ID: <20260116171914.298018-2-pedrodemargomes@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260116171914.298018-1-pedrodemargomes@gmail.com> References: <2025112003-afloat-squall-e39d@gregkh> <20260116171914.298018-1-pedrodemargomes@gmail.com> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit [ Upstream commit f5548c318d6520d4fa3c5ed6003eeb710763cbc5 ] Currently, scan_get_next_rmap_item() walks every page address in a VMA to locate mergeable pages. This becomes highly inefficient when scanning large virtual memory areas that contain mostly unmapped regions, causing ksmd to use large amount of cpu without deduplicating much pages. This patch replaces the per-address lookup with a range walk using walk_page_range(). The range walker allows KSM to skip over entire unmapped holes in a VMA, avoiding unnecessary lookups. This problem was previously discussed in [1]. Consider the following test program which creates a 32 TiB mapping in the virtual address space but only populates a single page: /* 32 TiB */ const size_t size = 32ul * 1024 * 1024 * 1024 * 1024; int main() { char *area = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_NORESERVE | MAP_PRIVATE | MAP_ANON, -1, 0); if (area == MAP_FAILED) { perror("mmap() failed\n"); return -1; } /* Populate a single page such that we get an anon_vma. */ *area = 0; /* Enable KSM. */ madvise(area, size, MADV_MERGEABLE); pause(); return 0; } $ ./ksm-sparse & $ echo 1 > /sys/kernel/mm/ksm/run Without this patch ksmd uses 100% of the cpu for a long time (more then 1 hour in my test machine) scanning all the 32 TiB virtual address space that contain only one mapped page. This makes ksmd essentially deadlocked not able to deduplicate anything of value. With this patch ksmd walks only the one mapped page and skips the rest of the 32 TiB virtual address space, making the scan fast using little cpu. Link: https://lkml.kernel.org/r/20251023035841.41406-1-pedrodemargomes@gmail.com Link: https://lkml.kernel.org/r/20251022153059.22763-1-pedrodemargomes@gmail.com Link: https://lore.kernel.org/linux-mm/423de7a3-1c62-4e72-8e79-19a6413e420c@redhat.com/ [1] Fixes: 31dbd01f3143 ("ksm: Kernel SamePage Merging") Signed-off-by: Pedro Demarchi Gomes Co-developed-by: David Hildenbrand Signed-off-by: David Hildenbrand Reported-by: craftfever Closes: https://lkml.kernel.org/r/020cf8de6e773bb78ba7614ef250129f11a63781@murena.io Suggested-by: David Hildenbrand Acked-by: David Hildenbrand Cc: Chengming Zhou Cc: xu xin Cc: Signed-off-by: Andrew Morton [ change folio to page, replace pmdp_get_lockless with pmd_read_atomic and pmdp_get with READ_ONCE(*pmdp) ] Signed-off-by: Pedro Demarchi Gomes --- mm/ksm.c | 115 ++++++++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 105 insertions(+), 10 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index a5716fdec1aa..48b76785db0d 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -38,6 +38,7 @@ #include #include #include +#include #include #include "internal.h" @@ -2214,6 +2215,89 @@ static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot, return rmap_item; } +struct ksm_next_page_arg { + struct page *page; + unsigned long addr; +}; + +static int ksm_next_page_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned long end, + struct mm_walk *walk) +{ + struct ksm_next_page_arg *private = walk->private; + struct vm_area_struct *vma = walk->vma; + pte_t *start_ptep = NULL, *ptep, pte; + struct mm_struct *mm = walk->mm; + struct page *page; + spinlock_t *ptl; + pmd_t pmd; + + if (ksm_test_exit(mm)) + return 0; + + cond_resched(); + + pmd = pmd_read_atomic(pmdp); + if (!pmd_present(pmd)) + return 0; + + if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && pmd_leaf(pmd)) { + ptl = pmd_lock(mm, pmdp); + pmd = READ_ONCE(*pmdp); + + if (!pmd_present(pmd)) { + goto not_found_unlock; + } else if (pmd_leaf(pmd)) { + page = vm_normal_page_pmd(vma, addr, pmd); + if (!page) + goto not_found_unlock; + + if (is_zone_device_page(page) || !PageAnon(page)) + goto not_found_unlock; + + page += ((addr & (PMD_SIZE - 1)) >> PAGE_SHIFT); + goto found_unlock; + } + spin_unlock(ptl); + } + + start_ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl); + if (!start_ptep) + return 0; + + for (ptep = start_ptep; addr < end; ptep++, addr += PAGE_SIZE) { + pte = ptep_get(ptep); + + if (!pte_present(pte)) + continue; + + page = vm_normal_page(vma, addr, pte); + if (!page) + continue; + + if (is_zone_device_page(page) || !PageAnon(page)) + continue; + goto found_unlock; + } + +not_found_unlock: + spin_unlock(ptl); + if (start_ptep) + pte_unmap(start_ptep); + return 0; +found_unlock: + get_page(page); + spin_unlock(ptl); + if (start_ptep) + pte_unmap(start_ptep); + private->page = page; + private->addr = addr; + return 1; +} + +static struct mm_walk_ops ksm_next_page_ops = { + .pmd_entry = ksm_next_page_pmd_entry, +}; + static struct rmap_item *scan_get_next_rmap_item(struct page **page) { struct mm_struct *mm; @@ -2293,29 +2377,40 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page) ksm_scan.address = vma->vm_end; while (ksm_scan.address < vma->vm_end) { + struct ksm_next_page_arg ksm_next_page_arg; + struct page *tmp_page = NULL; + int found; + if (ksm_test_exit(mm)) break; - *page = follow_page(vma, ksm_scan.address, FOLL_GET); - if (IS_ERR_OR_NULL(*page)) { - ksm_scan.address += PAGE_SIZE; - cond_resched(); - continue; + + found = walk_page_range_vma(vma, ksm_scan.address, + vma->vm_end, + &ksm_next_page_ops, + &ksm_next_page_arg); + + if (found > 0) { + tmp_page = ksm_next_page_arg.page; + ksm_scan.address = ksm_next_page_arg.addr; + } else { + VM_WARN_ON_ONCE(found < 0); + ksm_scan.address = vma->vm_end - PAGE_SIZE; } - if (PageAnon(*page)) { - flush_anon_page(vma, *page, ksm_scan.address); - flush_dcache_page(*page); + if (tmp_page) { + flush_anon_page(vma, tmp_page, ksm_scan.address); + flush_dcache_page(tmp_page); rmap_item = get_next_rmap_item(slot, ksm_scan.rmap_list, ksm_scan.address); if (rmap_item) { ksm_scan.rmap_list = &rmap_item->rmap_list; ksm_scan.address += PAGE_SIZE; + *page = tmp_page; } else - put_page(*page); + put_page(tmp_page); mmap_read_unlock(mm); return rmap_item; } - put_page(*page); ksm_scan.address += PAGE_SIZE; cond_resched(); } -- 2.43.0