Messages in this thread | | | Date | Wed, 9 Nov 2022 11:40:01 +0100 | Subject | Re: [PATCH v3 2/5] ksm: support unsharing zero pages placed by KSM | From | David Hildenbrand <> |
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On 21.10.22 14:54, David Hildenbrand wrote: > On 21.10.22 12:17, David Hildenbrand wrote: >> On 11.10.22 04:22, xu.xin.sc@gmail.com wrote: >>> From: xu xin <xu.xin16@zte.com.cn> >>> >>> use_zero_pages may be very useful, not just because of cache colouring >>> as described in doc, but also because use_zero_pages can accelerate >>> merging empty pages when there are plenty of empty pages (full of zeros) >>> as the time of page-by-page comparisons (unstable_tree_search_insert) is >>> saved. >>> >>> But when enabling use_zero_pages, madvise(addr, len, MADV_UNMERGEABLE) and >>> other ways (like write 2 to /sys/kernel/mm/ksm/run) to trigger unsharing >>> will *not* unshare the shared zeropage as placed by KSM (which may be >>> against the MADV_UNMERGEABLE documentation at least). >>> >>> To not blindly unshare all shared zero_pages in applicable VMAs, the patch >>> introduces a dedicated flag ZERO_PAGE_FLAG to mark the rmap_items of those >>> shared zero_pages. and guarantee that these rmap_items will be not freed >>> during the time of zero_pages not being writing, so we can only unshare >>> the *KSM-placed* zero_pages. >>> >>> The patch will not degrade the performance of use_zero_pages as it doesn't >>> change the way of merging empty pages in use_zero_pages's feature. >>> >>> Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring") >>> Reported-by: David Hildenbrand <david@redhat.com> >>> Cc: Claudio Imbrenda <imbrenda@linux.ibm.com> >>> Cc: Xuexin Jiang <jiang.xuexin@zte.com.cn> >>> Signed-off-by: xu xin <xu.xin16@zte.com.cn> >>> Co-developed-by: Xiaokai Ran <ran.xiaokai@zte.com.cn> >>> Signed-off-by: Xiaokai Ran <ran.xiaokai@zte.com.cn> >>> Co-developed-by: Yang Yang <yang.yang29@zte.com.cn> >>> Signed-off-by: Yang Yang <yang.yang29@zte.com.cn> >>> Signed-off-by: xu xin <xu.xin16@zte.com.cn> >>> --- >>> mm/ksm.c | 136 ++++++++++++++++++++++++++++++++++++++++++------------- >>> 1 file changed, 105 insertions(+), 31 deletions(-) >>> >>> diff --git a/mm/ksm.c b/mm/ksm.c >>> index 13c60f1071d8..e351d7b6d15e 100644 >>> --- a/mm/ksm.c >>> +++ b/mm/ksm.c >>> @@ -213,6 +213,7 @@ struct ksm_rmap_item { >>> #define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */ >>> #define UNSTABLE_FLAG 0x100 /* is a node of the unstable tree */ >>> #define STABLE_FLAG 0x200 /* is listed from the stable tree */ >>> +#define ZERO_PAGE_FLAG 0x400 /* is zero page placed by KSM */ >>> >>> /* The stable and unstable tree heads */ >>> static struct rb_root one_stable_tree[1] = { RB_ROOT }; >>> @@ -381,14 +382,6 @@ static inline struct ksm_rmap_item *alloc_rmap_item(void) >>> return rmap_item; >>> } >>> >>> -static inline void free_rmap_item(struct ksm_rmap_item *rmap_item) >>> -{ >>> - ksm_rmap_items--; >>> - rmap_item->mm->ksm_rmap_items--; >>> - rmap_item->mm = NULL; /* debug safety */ >>> - kmem_cache_free(rmap_item_cache, rmap_item); >>> -} >>> - >>> static inline struct ksm_stable_node *alloc_stable_node(void) >>> { >>> /* >>> @@ -420,7 +413,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm) >>> } >>> >>> /* >>> - * We use break_ksm to break COW on a ksm page: it's a stripped down >>> + * We use break_ksm to break COW on a ksm page or KSM-placed zero page (only >>> + * happen when enabling use_zero_pages): it's a stripped down >>> * >>> * if (get_user_pages(addr, 1, FOLL_WRITE, &page, NULL) == 1) >>> * put_page(page); >>> @@ -434,7 +428,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm) >>> * of the process that owns 'vma'. We also do not want to enforce >>> * protection keys here anyway. >>> */ >>> -static int break_ksm(struct vm_area_struct *vma, unsigned long addr) >>> +static int break_ksm(struct vm_area_struct *vma, unsigned long addr, >>> + bool ksm_check_bypass) >>> { >>> struct page *page; >>> vm_fault_t ret = 0; >>> @@ -449,6 +444,16 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr) >>> ret = handle_mm_fault(vma, addr, >>> FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE, >>> NULL); >>> + else if (ksm_check_bypass && is_zero_pfn(page_to_pfn(page))) { >>> + /* >>> + * Although it's not ksm page, it's zero page as placed by >>> + * KSM use_zero_page, so we should unshare it when >>> + * ksm_check_bypass is true. >>> + */ >>> + ret = handle_mm_fault(vma, addr, >>> + FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE, >>> + NULL); >>> + } >> >> Please don't duplicate that page fault triggering code. >> >> Also, please be aware that this collides with >> >> https://lkml.kernel.org/r/20221021101141.84170-1-david@redhat.com >> >> Adjustments should be comparatively easy. > > ... except that I'm still working on FAULT_FLAG_UNSHARE support for the > shared zeropage. That will be posted soonish (within next 2 weeks). >
Posted: https://lkml.kernel.org/r/20221107161740.144456-1-david@redhat.com
With that, we can use FAULT_FLAG_UNSHARE also to break COW on the shared zeropage.
-- Thanks,
David / dhildenb
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