From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f197.google.com (mail-pf1-f197.google.com [209.85.210.197]) by kanga.kvack.org (Postfix) with ESMTP id 05CD36B000C for ; Tue, 16 Oct 2018 07:43:20 -0400 (EDT) Received: by mail-pf1-f197.google.com with SMTP id a72-v6so7255538pfj.14 for ; Tue, 16 Oct 2018 04:43:19 -0700 (PDT) Received: from mga11.intel.com (mga11.intel.com. [192.55.52.93]) by mx.google.com with ESMTPS id j16-v6si13692745pgm.501.2018.10.16.04.43.18 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Oct 2018 04:43:18 -0700 (PDT) Date: Tue, 16 Oct 2018 14:43:12 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH RFC] ksm: Assist buddy allocator to assemble 1-order pages Message-ID: <20181016114312.bxkxxuttonfbjmsg@black.fi.intel.com> References: <153925511661.21256.9692370932417728663.stgit@localhost.localdomain> <20181015154112.6bj5p4zuxjtz43pd@kshutemo-mobl1> <0b0a81c4-d0b3-99f4-6910-10b757732825@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0b0a81c4-d0b3-99f4-6910-10b757732825@virtuozzo.com> Sender: owner-linux-mm@kvack.org List-ID: To: Kirill Tkhai Cc: "Kirill A. Shutemov" , akpm@linux-foundation.org, 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 Tue, Oct 16, 2018 at 09:34:11AM +0000, Kirill Tkhai wrote: > 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. Looks fine to me. -- Kirill A. Shutemov