Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754191AbbG2Nzf (ORCPT ); Wed, 29 Jul 2015 09:55:35 -0400 Received: from outbound-smtp02.blacknight.com ([81.17.249.8]:35799 "EHLO outbound-smtp02.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753557AbbG2Nze (ORCPT ); Wed, 29 Jul 2015 09:55:34 -0400 Date: Wed, 29 Jul 2015 14:55:25 +0100 From: Mel Gorman To: Vlastimil Babka Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "minkyung88.kim" , kmk3210@gmail.com, Seungho Park , Joonsoo Kim , Minchan Kim , Michal Nazarewicz , Laura Abbott , Naoya Horiguchi , Johannes Weiner , "Kirill A. Shutemov" Subject: Re: [PATCH 1/2] mm, page_isolation: remove bogus tests for isolated pages Message-ID: <20150729135525.GG19352@techsingularity.net> References: <55969822.9060907@suse.cz> <1437483218-18703-1-git-send-email-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <1437483218-18703-1-git-send-email-vbabka@suse.cz> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2281 Lines: 41 On Tue, Jul 21, 2015 at 02:53:37PM +0200, Vlastimil Babka wrote: > The __test_page_isolated_in_pageblock() is used to verify whether all pages > in pageblock were either successfully isolated, or are hwpoisoned. Two of the > possible state of pages, that are tested, are however bogus and misleading. > > Both tests rely on get_freepage_migratetype(page), which however has no > guarantees about pages on freelists. Specifically, it doesn't guarantee that > the migratetype returned by the function actually matches the migratetype of > the freelist that the page is on. Such guarantee is not its purpose and would > have negative impact on allocator performance. > > The first test checks whether the freepage_migratetype equals MIGRATE_ISOLATE, > supposedly to catch races between page isolation and allocator activity. These > races should be fixed nowadays with 51bb1a4093 ("mm/page_alloc: add freepage > on isolate pageblock to correct buddy list") and related patches. As explained > above, the check wouldn't be able to catch them reliably anyway. For the same > reason false positives can happen, although they are harmless, as the > move_freepages() call would just move the page to the same freelist it's > already on. So removing the test is not a bug fix, just cleanup. After this > patch, we assume that all PageBuddy pages are on the correct freelist and that > the races were really fixed. A truly reliable verification in the form of e.g. > VM_BUG_ON() would be complicated and is arguably not needed. > > The second test (page_count(page) == 0 && get_freepage_migratetype(page) > == MIGRATE_ISOLATE) is probably supposed (the code comes from a big memory > isolation patch from 2007) to catch pages on MIGRATE_ISOLATE pcplists. > However, pcplists don't contain MIGRATE_ISOLATE freepages nowadays, those are > freed directly to free lists, so the check is obsolete. Remove it as well. > > Signed-off-by: Vlastimil Babka Acked-by: Mel Gorman -- Mel Gorman SUSE Labs -- 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/