kernel/linux.git/include/linux/ksm.h, branch v6.12.80

mm/ksm: fix incorrect KSM counter handling in mm_struct during fork

2025-10-19T14:34:01+00:00

[ Upstream commit 4d6fc29f36341d7795db1d1819b4c15fe9be7b23 ] Patch series "mm/ksm: Fix incorrect accounting of KSM counters during fork", v3. The first patch in this series fixes the incorrect accounting of KSM counters such as ksm_merging_pages, ksm_rmap_items, and the global ksm_zero_pages during fork. The following patch add a selftest to verify the ksm_merging_pages counter was updated correctly during fork. Test Results ============ Without the first patch ----------------------- # [RUN] test_fork_ksm_merging_page_count not ok 10 ksm_merging_page in child: 32 With the first patch -------------------- # [RUN] test_fork_ksm_merging_page_count ok 10 ksm_merging_pages is not inherited after fork This patch (of 2): Currently, the KSM-related counters in `mm_struct`, such as `ksm_merging_pages`, `ksm_rmap_items`, and `ksm_zero_pages`, are inherited by the child process during fork. This results in inconsistent accounting. When a process uses KSM, identical pages are merged and an rmap item is created for each merged page. The `ksm_merging_pages` and `ksm_rmap_items` counters are updated accordingly. However, after a fork, these counters are copied to the child while the corresponding rmap items are not. As a result, when the child later triggers an unmerge, there are no rmap items present in the child, so the counters remain stale, leading to incorrect accounting. A similar issue exists with `ksm_zero_pages`, which maintains both a global counter and a per-process counter. During fork, the per-process counter is inherited by the child, but the global counter is not incremented. Since the child also references zero pages, the global counter should be updated as well. Otherwise, during zero-page unmerge, both the global and per-process counters are decremented, causing the global counter to become inconsistent. To fix this, ksm_merging_pages and ksm_rmap_items are reset to 0 during fork, and the global ksm_zero_pages counter is updated with the per-process ksm_zero_pages value inherited by the child. This ensures that KSM statistics remain accurate and reflect the activity of each process correctly. Link: https://lkml.kernel.org/r/cover.1758648700.git.donettom@linux.ibm.com Link: https://lkml.kernel.org/r/7b9870eb67ccc0d79593940d9dbd4a0b39b5d396.1758648700.git.donettom@linux.ibm.com Fixes: 7609385337a4 ("ksm: count ksm merging pages for each process") Fixes: cb4df4cae4f2 ("ksm: count allocated ksm rmap_items for each process") Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM") Signed-off-by: Donet Tom Reviewed-by: Chengming Zhou Acked-by: David Hildenbrand Cc: Aboorva Devarajan Cc: David Hildenbrand Cc: Donet Tom Cc: "Ritesh Harjani (IBM)" Cc: Wei Yang Cc: xu xin Cc: [6.6+] Signed-off-by: Andrew Morton [ replaced mm_flags_test() calls with test_bit() ] Signed-off-by: Sasha Levin Signed-off-by: Greg Kroah-Hartman

fork: only invoke khugepaged, ksm hooks if no error

2024-10-29T04:40:39+00:00

There is no reason to invoke these hooks early against an mm that is in an incomplete state. The change in commit d24062914837 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()") makes this more pertinent as we may be in a state where entries in the maple tree are not yet consistent. Their placement early in dup_mmap() only appears to have been meaningful for early error checking, and since functionally it'd require a very small allocation to fail (in practice 'too small to fail') that'd only occur in the most dire circumstances, meaning the fork would fail or be OOM'd in any case. Since both khugepaged and KSM tracking are there to provide optimisations to memory performance rather than critical functionality, it doesn't really matter all that much if, under such dire memory pressure, we fail to register an mm with these. As a result, we follow the example of commit d2081b2bf819 ("mm: khugepaged: make khugepaged_enter() void function") and make ksm_fork() a void function also. We only expose the mm to these functions once we are done with them and only if no error occurred in the fork operation. Link: https://lkml.kernel.org/r/e0cb8b840c9d1d5a6e84d4f8eff5f3f2022aa10c.1729014377.git.lorenzo.stoakes@oracle.com Fixes: d24062914837 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()") Signed-off-by: Lorenzo Stoakes Reported-by: Jann Horn Reviewed-by: Liam R. Howlett Reviewed-by: Vlastimil Babka Reviewed-by: Jann Horn Cc: Alexander Viro Cc: Christian Brauner Cc: Jan Kara Cc: Linus Torvalds Cc: Signed-off-by: Andrew Morton

mm/ksm: fix ksm_zero_pages accounting

2024-06-06T02:19:26+00:00

We normally ksm_zero_pages++ in ksmd when page is merged with zero page, but ksm_zero_pages-- is done from page tables side, where there is no any accessing protection of ksm_zero_pages. So we can read very exceptional value of ksm_zero_pages in rare cases, such as -1, which is very confusing to users. Fix it by changing to use atomic_long_t, and the same case with the mm->ksm_zero_pages. Link: https://lkml.kernel.org/r/20240528-b4-ksm-counters-v3-2-34bb358fdc13@linux.dev Fixes: e2942062e01d ("ksm: count all zero pages placed by KSM") Fixes: 6080d19f0704 ("ksm: add ksm zero pages for each process") Signed-off-by: Chengming Zhou Acked-by: David Hildenbrand Cc: Andrea Arcangeli Cc: Hugh Dickins Cc: Ran Xiaokai Cc: Stefan Roesch Cc: xu xin Cc: Yang Yang Cc: Signed-off-by: Andrew Morton

mm/memory-failure: pass the folio to collect_procs_ksm()

2024-05-06T00:53:47+00:00

We've already calculated it, so pass it in instead of recalculating it in collect_procs_ksm(). Link: https://lkml.kernel.org/r/20240412193510.2356957-12-willy@infradead.org Signed-off-by: Matthew Wilcox (Oracle) Reviewed-by: Jane Chu Reviewed-by: Miaohe Lin Cc: Dan Williams Cc: Miaohe Lin Cc: Oscar Salvador Signed-off-by: Andrew Morton

mm/ksm: remove redundant code in ksm_fork

2024-04-26T03:56:41+00:00

Since commit 3c6f33b7273a ("mm/ksm: support fork/exec for prctl"), when a child process is forked, the MMF_VM_MERGE_ANY flag will be inherited in mm_init(). So, it's unnecessary to set the flag in ksm_fork(). Link: https://lkml.kernel.org/r/20240402024934.1093361-1-tujinjiang@huawei.com Signed-off-by: Jinjiang Tu Reviewed-by: David Hildenbrand Cc: Johannes Weiner Cc: Kefeng Wang Cc: Nanyong Sun Cc: Rik van Riel Cc: Stefan Roesch Signed-off-by: Andrew Morton

mm/ksm: fix ksm exec support for prctl

2024-04-26T03:56:29+00:00

Patch series "mm/ksm: fix ksm exec support for prctl", v4. commit 3c6f33b7273a ("mm/ksm: support fork/exec for prctl") inherits MMF_VM_MERGE_ANY flag when a task calls execve(). However, it doesn't create the mm_slot, so ksmd will not try to scan this task. The first patch fixes the issue. The second patch refactors to prepare for the third patch. The third patch extends the selftests of ksm to verfity the deduplication really happens after fork/exec inherits ths KSM setting. This patch (of 3): commit 3c6f33b7273a ("mm/ksm: support fork/exec for prctl") inherits MMF_VM_MERGE_ANY flag when a task calls execve(). Howerver, it doesn't create the mm_slot, so ksmd will not try to scan this task. To fix it, allocate and add the mm_slot to ksm_mm_head in __bprm_mm_init() when the mm has MMF_VM_MERGE_ANY flag. Link: https://lkml.kernel.org/r/20240328111010.1502191-1-tujinjiang@huawei.com Link: https://lkml.kernel.org/r/20240328111010.1502191-2-tujinjiang@huawei.com Fixes: 3c6f33b7273a ("mm/ksm: support fork/exec for prctl") Signed-off-by: Jinjiang Tu Reviewed-by: David Hildenbrand Cc: Johannes Weiner Cc: Kefeng Wang Cc: Nanyong Sun Cc: Rik van Riel Cc: Stefan Roesch Signed-off-by: Andrew Morton

mm: convert ksm_might_need_to_copy() to work on folios

2023-12-29T19:58:25+00:00

Patch series "Finish two folio conversions". Most callers of page_add_new_anon_rmap() and lru_cache_add_inactive_or_unevictable() have been converted to their folio equivalents, but there are still a few stragglers. There's a bit of preparatory work in ksm and unuse_pte(), but after that it's pretty mechanical. This patch (of 9): Accept a folio as an argument and return a folio result. Removes a call to compound_head() in do_swap_page(), and prevents folio & page from getting out of sync in unuse_pte(). Reviewed-by: David Hildenbrand [willy@infradead.org: fix smatch warning] Link: https://lkml.kernel.org/r/ZXnPtblC6A1IkyAB@casper.infradead.org [david@redhat.com: only adjust the page if the folio changed] Link: https://lkml.kernel.org/r/6a8f2110-fa91-4c10-9eae-88315309a6e3@redhat.com Link: https://lkml.kernel.org/r/20231211162214.2146080-1-willy@infradead.org Link: https://lkml.kernel.org/r/20231211162214.2146080-2-willy@infradead.org Signed-off-by: Matthew Wilcox (Oracle) Signed-off-by: David Hildenbrand Signed-off-by: Andrew Morton

mm: ksm: use more folio api in ksm_might_need_to_copy()

2023-12-12T18:57:05+00:00

Patch series "mm: cleanup and use more folio in page fault", v3. Rename page_copy_prealloc() to folio_prealloc(), which is used by more functions, also do more folio conversion in page fault. This patch (of 5): Since ksm only support normal page, no swapout/in for ksm large folio too, add large folio check in ksm_might_need_to_copy(), also convert page->index to folio->index as page->index is going away. Then convert ksm_might_need_to_copy() to use more folio api to save nine compound_head() calls, short 'address' to reduce max-line-length. Link: https://lkml.kernel.org/r/20231118023232.1409103-1-wangkefeng.wang@huawei.com Link: https://lkml.kernel.org/r/20231118023232.1409103-2-wangkefeng.wang@huawei.com Signed-off-by: Kefeng Wang Cc: David Hildenbrand Cc: Matthew Wilcox (Oracle) Cc: Sidhartha Kumar Cc: Vishal Moola (Oracle) Signed-off-by: Andrew Morton

ksm: add ksm zero pages for each process

2023-08-18T17:12:10+00:00

As the number of ksm zero pages is not included in ksm_merging_pages per process when enabling use_zero_pages, it's unclear of how many actual pages are merged by KSM. To let users accurately estimate their memory demands when unsharing KSM zero-pages, it's necessary to show KSM zero- pages per process. In addition, it help users to know the actual KSM profit because KSM-placed zero pages are also benefit from KSM. since unsharing zero pages placed by KSM accurately is achieved, then tracking empty pages merging and unmerging is not a difficult thing any longer. Since we already have /proc//ksm_stat, just add the information of 'ksm_zero_pages' in it. Link: https://lkml.kernel.org/r/20230613030938.185993-1-yang.yang29@zte.com.cn Signed-off-by: xu xin Acked-by: David Hildenbrand Reviewed-by: Xiaokai Ran Reviewed-by: Yang Yang Cc: Claudio Imbrenda Cc: Xuexin Jiang Signed-off-by: Andrew Morton

ksm: count all zero pages placed by KSM

2023-08-18T17:12:09+00:00

As pages_sharing and pages_shared don't include the number of zero pages merged by KSM, we cannot know how many pages are zero pages placed by KSM when enabling use_zero_pages, which leads to KSM not being transparent with all actual merged pages by KSM. In the early days of use_zero_pages, zero-pages was unable to get unshared by the ways like MADV_UNMERGEABLE so it's hard to count how many times one of those zeropages was then unmerged. But now, unsharing KSM-placed zero page accurately has been achieved, so we can easily count both how many times a page full of zeroes was merged with zero-page and how many times one of those pages was then unmerged. and so, it helps to estimate memory demands when each and every shared page could get unshared. So we add ksm_zero_pages under /sys/kernel/mm/ksm/ to show the number of all zero pages placed by KSM. Meanwhile, we update the Documentation. Link: https://lkml.kernel.org/r/20230613030934.185944-1-yang.yang29@zte.com.cn Signed-off-by: xu xin Acked-by: David Hildenbrand Cc: Claudio Imbrenda Cc: Xuexin Jiang Reviewed-by: Xiaokai Ran Reviewed-by: Yang Yang Signed-off-by: Andrew Morton