From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lj1-f197.google.com (mail-lj1-f197.google.com [209.85.208.197]) by kanga.kvack.org (Postfix) with ESMTP id 9FC696B2FC7 for ; Fri, 23 Nov 2018 02:03:11 -0500 (EST) Received: by mail-lj1-f197.google.com with SMTP id t7-v6so3211312ljg.9 for ; Thu, 22 Nov 2018 23:03:11 -0800 (PST) Received: from relay.sw.ru (relay.sw.ru. [185.231.240.75]) by mx.google.com with ESMTPS id y7-v6si26064497ljj.102.2018.11.22.23.03.09 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 22 Nov 2018 23:03:09 -0800 (PST) Subject: Re: [PATCH 2/2] mm: ksm: do not block on page lock when searching stable tree References: <1541618201-120667-1-git-send-email-yang.shi@linux.alibaba.com> <1541618201-120667-2-git-send-email-yang.shi@linux.alibaba.com> From: Kirill Tkhai Message-ID: <302f3116-7e53-2750-ec2d-b90feb4cafec@virtuozzo.com> Date: Fri, 23 Nov 2018 10:03:04 +0300 MIME-Version: 1.0 In-Reply-To: <1541618201-120667-2-git-send-email-yang.shi@linux.alibaba.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Yang Shi , mhocko@kernel.org, vbabka@suse.cz, hannes@cmpxchg.org, hughd@google.com, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org On 07.11.2018 22:16, Yang Shi wrote: > ksmd need search stable tree to look for the suitable KSM page, but the > KSM page might be locked for long time due to i.e. KSM page rmap walk. > > It sounds not worth waiting for the lock, the page can be skip, then try > to merge it in the next scan to avoid long stall if its content is > still intact. > > Introduce async mode to get_ksm_page() to not block on page lock, like > what try_to_merge_one_page() does. > > Return -EBUSY if trylock fails, since NULL means not find suitable KSM > page, which is a valid case. > > Signed-off-by: Yang Shi Reviewed-by: Kirill Tkhai > --- > mm/ksm.c | 29 +++++++++++++++++++++++++---- > 1 file changed, 25 insertions(+), 4 deletions(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 5b0894b..576803d 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -667,7 +667,7 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node) > } > > /* > - * get_ksm_page: checks if the page indicated by the stable node > + * __get_ksm_page: checks if the page indicated by the stable node > * is still its ksm page, despite having held no reference to it. > * In which case we can trust the content of the page, and it > * returns the gotten page; but if the page has now been zapped, > @@ -685,7 +685,8 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node) > * a page to put something that might look like our key in page->mapping. > * is on its way to being freed; but it is an anomaly to bear in mind. > */ > -static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it) > +static struct page *__get_ksm_page(struct stable_node *stable_node, > + bool lock_it, bool async) > { > struct page *page; > void *expected_mapping; > @@ -728,7 +729,14 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it) > } > > if (lock_it) { > - lock_page(page); > + if (async) { > + if (!trylock_page(page)) { > + put_page(page); > + return ERR_PTR(-EBUSY); > + } > + } else > + lock_page(page); > + > if (READ_ONCE(page->mapping) != expected_mapping) { > unlock_page(page); > put_page(page); > @@ -751,6 +759,11 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it) > return NULL; > } > > +static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it) > +{ > + return __get_ksm_page(stable_node, lock_it, false); > +} > + > /* > * Removing rmap_item from stable or unstable tree. > * This function will clean the information from the stable/unstable tree. > @@ -1675,7 +1688,11 @@ static struct page *stable_tree_search(struct page *page) > * It would be more elegant to return stable_node > * than kpage, but that involves more changes. > */ > - tree_page = get_ksm_page(stable_node_dup, true); > + tree_page = __get_ksm_page(stable_node_dup, true, true); > + > + if (PTR_ERR(tree_page) == -EBUSY) > + return ERR_PTR(-EBUSY); > + > if (unlikely(!tree_page)) > /* > * The tree may have been rebalanced, > @@ -2062,6 +2079,10 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item) > > /* We first start with searching the page inside the stable tree */ > kpage = stable_tree_search(page); > + > + if (PTR_ERR(kpage) == -EBUSY) > + return; > + > if (kpage == page && rmap_item->head == stable_node) { > put_page(kpage); > return; >