Messages in this thread | | | Date | Fri, 21 Oct 2022 14:54:22 +0200 | 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 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).
-- Thanks,
David / dhildenb
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