From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f49.google.com (mail-pa0-f49.google.com [209.85.220.49]) by kanga.kvack.org (Postfix) with ESMTP id B8CB06B02B7 for ; Tue, 21 Jul 2015 18:43:45 -0400 (EDT) Received: by pachj5 with SMTP id hj5so127326984pac.3 for ; Tue, 21 Jul 2015 15:43:45 -0700 (PDT) Received: from mail-pa0-x234.google.com (mail-pa0-x234.google.com. [2607:f8b0:400e:c03::234]) by mx.google.com with ESMTPS id tv5si12237798pac.203.2015.07.21.15.43.44 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 21 Jul 2015 15:43:44 -0700 (PDT) Received: by padck2 with SMTP id ck2so126704752pad.0 for ; Tue, 21 Jul 2015 15:43:44 -0700 (PDT) Date: Tue, 21 Jul 2015 15:43:42 -0700 (PDT) From: David Rientjes Subject: Re: [PATCH 1/2] mm, page_isolation: remove bogus tests for isolated pages In-Reply-To: <1437483218-18703-1-git-send-email-vbabka@suse.cz> Message-ID: 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 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 , Joonsoo Kim , Minchan Kim , Michal Nazarewicz , Laura Abbott , Naoya Horiguchi , Johannes Weiner , "Kirill A. Shutemov" , Mel Gorman On Tue, 21 Jul 2015, 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 > Cc: Joonsoo Kim > Cc: Minchan Kim > Cc: Michal Nazarewicz > Cc: Laura Abbott > Cc: Naoya Horiguchi > --- > mm/page_isolation.c | 30 ++++++------------------------ > 1 file changed, 6 insertions(+), 24 deletions(-) > > diff --git a/mm/page_isolation.c b/mm/page_isolation.c > index 0e69d25..9eaa489c 100644 > --- a/mm/page_isolation.c > +++ b/mm/page_isolation.c > @@ -226,34 +226,16 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn, > continue; > } > page = pfn_to_page(pfn); > - if (PageBuddy(page)) { > + if (PageBuddy(page)) > /* > - * If race between isolatation and allocation happens, > - * some free pages could be in MIGRATE_MOVABLE list > - * although pageblock's migratation type of the page > - * is MIGRATE_ISOLATE. Catch it and move the page into > - * MIGRATE_ISOLATE list. > + * If the page is on a free list, it has to be on > + * the correct MIGRATE_ISOLATE freelist. There is no > + * simple way to verify that as VM_BUG_ON(), though. > */ > - if (get_freepage_migratetype(page) != MIGRATE_ISOLATE) { > - struct page *end_page; > - > - end_page = page + (1 << page_order(page)) - 1; > - move_freepages(page_zone(page), page, end_page, > - MIGRATE_ISOLATE); > - } > pfn += 1 << page_order(page); > - } > - else if (page_count(page) == 0 && > - get_freepage_migratetype(page) == MIGRATE_ISOLATE) > - pfn += 1; > - else if (skip_hwpoisoned_pages && PageHWPoison(page)) { > - /* > - * The HWPoisoned page may be not in buddy > - * system, and page_count() is not 0. > - */ > + else if (skip_hwpoisoned_pages && PageHWPoison(page)) > + /* A HWPoisoned page cannot be also PageBuddy */ > pfn++; > - continue; > - } > else > break; > } You may want to consider stating your assumptions explicitly in the code, perhaps with VM_BUG_ON(), such as in free_pcppages_bulk() to ensure things like get_freepage_migratetype(page) != MIGRATE_ISOLATE. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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