From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f170.google.com (mail-pd0-f170.google.com [209.85.192.170]) by kanga.kvack.org (Postfix) with ESMTP id EC4C56B025A for ; Thu, 23 Jul 2015 01:18:50 -0400 (EDT) Received: by pdrg1 with SMTP id g1so150884323pdr.2 for ; Wed, 22 Jul 2015 22:18:50 -0700 (PDT) Received: from lgemrelse7q.lge.com (LGEMRELSE7Q.lge.com. [156.147.1.151]) by mx.google.com with ESMTP id vf13si9006371pac.178.2015.07.22.22.18.49 for ; Wed, 22 Jul 2015 22:18:50 -0700 (PDT) Date: Thu, 23 Jul 2015 14:23:14 +0900 From: Joonsoo Kim Subject: Re: [PATCH 1/2] mm, page_isolation: remove bogus tests for isolated pages Message-ID: <20150723052314.GC4449@js1304-P5Q-DELUXE> References: <55969822.9060907@suse.cz> <1437483218-18703-1-git-send-email-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1437483218-18703-1-git-send-email-vbabka@suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "minkyung88.kim" , kmk3210@gmail.com, Seungho Park , Minchan Kim , Michal Nazarewicz , Laura Abbott , Naoya Horiguchi , Johannes Weiner , "Kirill A. Shutemov" , Mel Gorman 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: Joonsoo Kim Thanks for taking care of this. Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org