From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lj1-f200.google.com (mail-lj1-f200.google.com [209.85.208.200]) by kanga.kvack.org (Postfix) with ESMTP id 10B5C6B0006 for ; Tue, 16 Oct 2018 05:34:26 -0400 (EDT) Received: by mail-lj1-f200.google.com with SMTP id h7-v6so6265296ljc.15 for ; Tue, 16 Oct 2018 02:34:25 -0700 (PDT) Received: from relay.sw.ru (relay.sw.ru. [185.231.240.75]) by mx.google.com with ESMTPS id c18-v6si11079475lja.167.2018.10.16.02.34.23 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Oct 2018 02:34:23 -0700 (PDT) Subject: Re: [PATCH RFC] ksm: Assist buddy allocator to assemble 1-order pages References: <153925511661.21256.9692370932417728663.stgit@localhost.localdomain> <20181015154112.6bj5p4zuxjtz43pd@kshutemo-mobl1> From: Kirill Tkhai Message-ID: <0b0a81c4-d0b3-99f4-6910-10b757732825@virtuozzo.com> Date: Tue, 16 Oct 2018 12:34:11 +0300 MIME-Version: 1.0 In-Reply-To: <20181015154112.6bj5p4zuxjtz43pd@kshutemo-mobl1> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, andriy.shevchenko@linux.intel.com, mhocko@suse.com, rppt@linux.vnet.ibm.com, imbrenda@linux.vnet.ibm.com, corbet@lwn.net, ndesaulniers@google.com, dave.jiang@intel.com, jglisse@redhat.com, jia.he@hxt-semitech.com, paulmck@linux.vnet.ibm.com, colin.king@canonical.com, jiang.biao2@zte.com.cn, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 15.10.2018 18:41, Kirill A. Shutemov wrote: > On Thu, Oct 11, 2018 at 01:52:22PM +0300, Kirill Tkhai wrote: >> try_to_merge_two_pages() merges two pages, one of them >> is a page of currently scanned mm, the second is a page >> with identical hash from unstable tree. Currently, we >> merge the page from unstable tree into the first one, >> and then free it. >> >> The idea of this patch is to prefer freeing that page >> of them, which has a free neighbour (i.e., neighbour >> with zero page_count()). This allows buddy allocator >> to assemble at least 1-order set from the freed page >> and its neighbour; this is a kind of cheep passive >> compaction. >> >> AFAIK, 1-order pages set consists of pages with PFNs >> [2n, 2n+1] (odd, even), so the neighbour's pfn is >> calculated via XOR with 1. We check the result pfn >> is valid and its page_count(), and prefer merging >> into @tree_page if neighbour's usage count is zero. >> >> There a is small difference with current behavior >> in case of error path. In case of the second >> try_to_merge_with_ksm_page() is failed, we return >> from try_to_merge_two_pages() with @tree_page >> removed from unstable tree. It does not seem to matter, >> but if we do not want a change at all, it's not >> a problem to move remove_rmap_item_from_tree() from >> try_to_merge_with_ksm_page() to its callers. >> >> Signed-off-by: Kirill Tkhai >> --- >> mm/ksm.c | 15 +++++++++++++++ >> 1 file changed, 15 insertions(+) >> >> diff --git a/mm/ksm.c b/mm/ksm.c >> index 5b0894b45ee5..b83ca37e28f0 100644 >> --- a/mm/ksm.c >> +++ b/mm/ksm.c >> @@ -1321,6 +1321,21 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item, >> { >> int err; >> >> + if (IS_ENABLED(CONFIG_COMPACTION)) { >> + unsigned long pfn; >> + /* >> + * Find neighbour of @page containing 1-order pair >> + * in buddy-allocator and check whether it is free. > > You cannot really check if the page is free. There are some paths that > makes the refcount zero temporarely, but doesn't free the page. > See page_ref_freeze() for instance. Thanks. Does this look better? Find neighbour of @page containing 1-order pair in buddy-allocator and check whether its count is 0. If it is so, we consider it's as free (this is more probable than it's freezed via page_ref_freeze()), and we try to use @tree_page as ksm page and to free @page. > It should be fine for the use case, but comment should state that we > speculate about page usage, not having definetive answer. > > [ I don't know enough about KSM to ack the patch in general, but it looks > fine to me at the first glance.] > >> + * If it is so, try to use @tree_page as ksm page >> + * and to free @page. >> + */ >> + pfn = (page_to_pfn(page) ^ 1); >> + if (pfn_valid(pfn) && page_count(pfn_to_page(pfn)) == 0) { >> + swap(rmap_item, tree_rmap_item); >> + swap(page, tree_page); >> + } >> + } >> + >> err = try_to_merge_with_ksm_page(rmap_item, page, NULL); >> if (!err) { >> err = try_to_merge_with_ksm_page(tree_rmap_item, >> >