From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f199.google.com (mail-pf1-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id BD5448E01DC for ; Fri, 14 Dec 2018 13:07:03 -0500 (EST) Received: by mail-pf1-f199.google.com with SMTP id p9so5005944pfj.3 for ; Fri, 14 Dec 2018 10:07:03 -0800 (PST) Received: from out30-133.freemail.mail.aliyun.com (out30-133.freemail.mail.aliyun.com. [115.124.30.133]) by mx.google.com with ESMTPS id w2si4569344pgh.565.2018.12.14.10.07.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 14 Dec 2018 10:07:01 -0800 (PST) Subject: Re: [PATCH] mm: Reuse only-pte-mapped KSM page in do_wp_page() References: <154471491016.31352.1168978849911555609.stgit@localhost.localdomain> <5d5bfbd2-8411-e707-1628-18bde66a6793@linux.alibaba.com> From: Yang Shi Message-ID: <00af5cd2-e226-89e3-3506-de5e6de05060@linux.alibaba.com> Date: Fri, 14 Dec 2018 10:06:42 -0800 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: Kirill Tkhai , akpm@linux-foundation.org, kirill@shutemov.name, hughd@google.com, aarcange@redhat.com Cc: christian.koenig@amd.com, imbrenda@linux.vnet.ibm.com, riel@surriel.com, ying.huang@intel.com, minchan@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org On 12/14/18 1:26 AM, Kirill Tkhai wrote: > On 13.12.2018 22:15, Yang Shi wrote: >> >> 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 >>> --- >>>   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? > Yes, this is the thing that the patch makes. Let's look at the actions, > we have without the patch, when there is a writing to an only-pte-mapped > KSM page. > > We enter to do_wp_page() with page_count() == 1, since KSM page is mapped > in only pte (and we do not get extra reference to a page, when we add it > to KSM stable tree). Then: > > do_wp_page() > get_page(vmf->page) <- page_count() is 2 > wp_page_copy() > .. > cow_user_page() /* Copy user page to a new one */ > .. > put_page(vmf->page) <- page_count() is 1 > put_page(vmf->page) <- page_count() is 0 > > Second put_page() frees the page (and also zeroes page->mapping), > and since that it's not a PageKsm() page anymore. Further > __get_ksm_page() calls will fail on this page (since the mapping > was zeroed), and its node will be unlinked from ksm stable tree: > > __get_ksm_page() > { > /* page->mapping == NULL, expected_mapping != NULL */ > if (READ_ONCE(page->mapping) != expected_mapping) > goto stale; > ....... > stale: > remove_node_from_stable_tree(stable_node); > } > > > The patch optimizes do_wp_page(), and makes it to avoid the copying > (like we have for ordinary anon pages). Since KSM page is freed anyway, > after we dropped the last reference to it; we reuse it instead of this. > So, the thing will now work in this way: > > do_wp_page() > lock_page(vmf->page) > reuse_ksm_page() > check PageSwapCache() and page_stable_node() > page_ref_freeze(page, 1) <- Freeze the page to make parallel > __get_ksm_page() (if any) waiting > page_move_anon_rmap() <- Write new mapping, so __get_ksm_page() > sees this is not a KSM page anymore, > and it removes stable node. > > So, the result is the same, but after the patch we achieve it faster :) > > Also, note, that in the most probably case, do_wp_page() does not cross > with __get_ksm_page() (the race window is very small; __get_ksm_page() > is spinning, only when reuse_ksm_page() is between page_ref_freeze() > and page_move_anon_rmap(), which are on neighboring lines). > > So, this is the idea. Please, let me know in case of something is unclear > for you. Thanks for elaborating this. It sounds reasonable. You can add Reviewed-by: Yang Shi > > Kirill