Messages in this thread | | | Date | Fri, 21 Oct 2022 12:17:30 +0200 | Subject | Re: [PATCH v3 2/5] ksm: support unsharing zero pages placed by KSM | From | David Hildenbrand <> |
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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.
-- Thanks,
David / dhildenb
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