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SubjectRe: [PATCH v3 2/5] ksm: support unsharing zero pages placed by KSM
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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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