From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f198.google.com (mail-pf1-f198.google.com [209.85.210.198]) by kanga.kvack.org (Postfix) with ESMTP id 1D73A6B000A for ; Thu, 11 Oct 2018 13:12:17 -0400 (EDT) Received: by mail-pf1-f198.google.com with SMTP id f4-v6so8790940pff.2 for ; Thu, 11 Oct 2018 10:12:17 -0700 (PDT) Received: from mga09.intel.com (mga09.intel.com. [134.134.136.24]) by mx.google.com with ESMTPS id v4-v6si28802045plb.400.2018.10.11.10.12.15 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 11 Oct 2018 10:12:15 -0700 (PDT) Date: Thu, 11 Oct 2018 20:07:45 +0300 From: Andy Shevchenko Subject: Re: [PATCH RFC] ksm: Assist buddy allocator to assemble 1-order pages Message-ID: <20181011170745.GN15943@smile.fi.intel.com> References: <153925511661.21256.9692370932417728663.stgit@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <153925511661.21256.9692370932417728663.stgit@localhost.localdomain> Sender: owner-linux-mm@kvack.org List-ID: To: Kirill Tkhai Cc: akpm@linux-foundation.org, kirill.shutemov@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 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; + blank line? > + /* > + * Find neighbour of @page containing 1-order pair > + * in buddy-allocator and check whether it is free. > + * If it is so, try to use @tree_page as ksm page > + * and to free @page. > + */ > + pfn = (page_to_pfn(page) ^ 1); Parens are not needed. > + 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, > -- With Best Regards, Andy Shevchenko