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Date: Thu, 13 Dec 2018 11:15:04 -0800
From: Yang Shi <yang.shi@...ux.alibaba.com>
To: Kirill Tkhai <ktkhai@...tuozzo.com>, akpm@...ux-foundation.org,
kirill@...temov.name, hughd@...gle.com, aarcange@...hat.com
Cc: christian.koenig@....com, imbrenda@...ux.vnet.ibm.com,
riel@...riel.com, ying.huang@...el.com, minchan@...nel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: Reuse only-pte-mapped KSM page in do_wp_page()
On 12/13/18 7:29 AM, Kirill Tkhai wrote:
> This patch adds an optimization for KSM pages almost
> in the same way, that we have for ordinary anonymous
> pages. If there is a write fault in a page, which is
> mapped to an only pte, and it is not related to swap
> cache; the page may be reused without copying its
> content.
>
> [Note, that we do not consider PageSwapCache() pages
> at least for now, since we don't want to complicate
> __get_ksm_page(), which has nice optimization based
> on this (for the migration case). Currenly it is
> spinning on PageSwapCache() pages, waiting for when
> they have unfreezed counters (i.e., for the migration
> finish). But we don't want to make it also spinning
> on swap cache pages, which we try to reuse, since
> there is not a very high probability to reuse them.
> So, for now we do not consider PageSwapCache() pages
> at all.]
>
> So, in reuse_ksm_page() we check for 1)PageSwapCache()
> and 2)page_stable_node(), to skip a page, which KSM
> is currently trying to link to stable tree. Then we
> do page_ref_freeze() to prohibit KSM to merge one more
> page into the page, we are reusing. After that, nobody
> can refer to the reusing page: KSM skips !PageSwapCache()
> pages with zero refcount; and the protection against
> of all other participants is the same as for reused
> ordinary anon pages pte lock, page lock and mmap_sem.
>
> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> ---
> include/linux/ksm.h | 7 +++++++
> mm/ksm.c | 25 +++++++++++++++++++++++--
> mm/memory.c | 16 ++++++++++++++--
> 3 files changed, 44 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/ksm.h b/include/linux/ksm.h
> index 161e8164abcf..e48b1e453ff5 100644
> --- a/include/linux/ksm.h
> +++ b/include/linux/ksm.h
> @@ -53,6 +53,8 @@ struct page *ksm_might_need_to_copy(struct page *page,
>
> void rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc);
> void ksm_migrate_page(struct page *newpage, struct page *oldpage);
> +bool reuse_ksm_page(struct page *page,
> + struct vm_area_struct *vma, unsigned long address);
>
> #else /* !CONFIG_KSM */
>
> @@ -86,6 +88,11 @@ static inline void rmap_walk_ksm(struct page *page,
> static inline void ksm_migrate_page(struct page *newpage, struct page *oldpage)
> {
> }
> +static inline bool reuse_ksm_page(struct page *page,
> + struct vm_area_struct *vma, unsigned long address)
> +{
> + return false;
> +}
> #endif /* CONFIG_MMU */
> #endif /* !CONFIG_KSM */
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 383f961e577a..fbd14264d784 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -707,8 +707,9 @@ static struct page *__get_ksm_page(struct stable_node *stable_node,
> * case this node is no longer referenced, and should be freed;
> * however, it might mean that the page is under page_ref_freeze().
> * The __remove_mapping() case is easy, again the node is now stale;
> - * but if page is swapcache in migrate_page_move_mapping(), it might
> - * still be our page, in which case it's essential to keep the node.
> + * the same is in reuse_ksm_page() case; but if page is swapcache
> + * in migrate_page_move_mapping(), it might still be our page,
> + * in which case it's essential to keep the node.
> */
> while (!get_page_unless_zero(page)) {
> /*
> @@ -2666,6 +2667,26 @@ void rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc)
> goto again;
> }
>
> +bool reuse_ksm_page(struct page *page,
> + struct vm_area_struct *vma,
> + unsigned long address)
> +{
> + VM_BUG_ON_PAGE(is_zero_pfn(page_to_pfn(page)), page);
> + VM_BUG_ON_PAGE(!page_mapped(page), page);
> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> +
> + if (PageSwapCache(page) || !page_stable_node(page))
> + return false;
> + /* Prohibit parallel get_ksm_page() */
> + if (!page_ref_freeze(page, 1))
> + return false;
> +
> + page_move_anon_rmap(page, vma);
Once the mapping is changed, it is not KSM mapping anymore. It looks
later get_ksm_page() would always fail on this page. Is this expected?
Thanks,
Yang
> + page->index = linear_page_index(vma, address);
> + page_ref_unfreeze(page, 1);
> +
> + return true;
> +}
> #ifdef CONFIG_MIGRATION
> void ksm_migrate_page(struct page *newpage, struct page *oldpage)
> {
> diff --git a/mm/memory.c b/mm/memory.c
> index 532061217e03..5817527f1877 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2509,8 +2509,11 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> * Take out anonymous pages first, anonymous shared vmas are
> * not dirty accountable.
> */
> - if (PageAnon(vmf->page) && !PageKsm(vmf->page)) {
> + if (PageAnon(vmf->page)) {
> int total_map_swapcount;
> + if (PageKsm(vmf->page) && (PageSwapCache(vmf->page) ||
> + page_count(vmf->page) != 1))
> + goto copy;
> if (!trylock_page(vmf->page)) {
> get_page(vmf->page);
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> @@ -2525,6 +2528,15 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> }
> put_page(vmf->page);
> }
> + if (PageKsm(vmf->page)) {
> + bool reused = reuse_ksm_page(vmf->page, vmf->vma,
> + vmf->address);
> + unlock_page(vmf->page);
> + if (!reused)
> + goto copy;
> + wp_page_reuse(vmf);
> + return VM_FAULT_WRITE;
> + }
> if (reuse_swap_page(vmf->page, &total_map_swapcount)) {
> if (total_map_swapcount == 1) {
> /*
> @@ -2545,7 +2557,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> (VM_WRITE|VM_SHARED))) {
> return wp_page_shared(vmf);
> }
> -
> +copy:
> /*
> * Ok, we need to copy. Oh, well..
> */
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