Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752410AbbBSAIh (ORCPT ); Wed, 18 Feb 2015 19:08:37 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58896 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751646AbbBSAIg (ORCPT ); Wed, 18 Feb 2015 19:08:36 -0500 Message-ID: <54E5296C.5040806@redhat.com> Date: Wed, 18 Feb 2015 19:08:12 -0500 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: Andrew Morton , Ebru Akagunduz CC: linux-mm@kvack.org, kirill@shutemov.name, mhocko@suse.cz, mgorman@suse.de, rientjes@google.com, sasha.levin@oracle.com, hughd@google.com, hannes@cmpxchg.org, vbabka@suse.cz, linux-kernel@vger.kernel.org, aarcange@redhat.com Subject: Re: [PATCH v2] mm: incorporate zero pages into transparent huge pages References: <1423688635-4306-1-git-send-email-ebru.akagunduz@gmail.com> <20150218153119.0bcd0bf8b4e7d30d99f00a3b@linux-foundation.org> In-Reply-To: <20150218153119.0bcd0bf8b4e7d30d99f00a3b@linux-foundation.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2136 Lines: 54 -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 02/18/2015 06:31 PM, Andrew Morton wrote: > On Wed, 11 Feb 2015 23:03:55 +0200 Ebru Akagunduz > wrote: > >> This patch improves THP collapse rates, by allowing zero pages. >> >> Currently THP can collapse 4kB pages into a THP when there are up >> to khugepaged_max_ptes_none pte_none ptes in a 2MB range. This >> patch counts pte none and mapped zero pages with the same >> variable. > > So if I'm understanding this correctly, with the default value of > khugepaged_max_ptes_none (HPAGE_PMD_NR-1), if an application > creates a 2MB area which contains 511 mappings of the zero page and > one real page, the kernel will proceed to turn that area into a > real, physical huge page. So it consumes 2MB of memory which would > not have previously been allocated? This is equivalent to an application doing a write fault to a 2MB area that was previously untouched, going into do_huge_pmd_anonymous_page() and receiving a 2MB page. > If so, this might be rather undesirable behaviour in some > situations (and ditto the current behaviour for pte_none ptes)? > > This can be tuned by adjusting khugepaged_max_ptes_none, The example of directly going into do_huge_pmd_anonymous_page() is not influenced by the tunable. It may indeed be undesirable in some situations, but I am not sure how to detect those... - -- All rights reversed -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJU5SlsAAoJEM553pKExN6D8DYH/0TQPr38R3lYqxTllOVPIUus +UrgXveOeoMiMbN3e5r9tIJkw+2yUJFZ8hkYx+aFsTD5zNz7xwf9Qz8IdJpcZ3sc PkvOnnZNk/ZzixWrBhWFPsKRN2pi5wXMpfNM2jTs9W4EeyfkV3RYbGxZy/OO1LB5 CwDzteCTb81y1FYxC4vNxLnML417ZjIMq7ICdj6lKW2KC5+TdCIPTOrKCy+2fWBo 4qhqho4RFKHLCxpnryUMzZDXca4vmcgGWwUm5xLF6SnJWWFEiPBLixJiRV3xe0iw rbuGhcIXo/q16oO4QOIl+hSVJr8vE+Y8xRbIJFmWXCmuQHQpg5ZspVZ+9Z/3UaI= =Qf1D -----END PGP SIGNATURE----- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/