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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id q68sm3761777ooa.29.2020.05.23.18.01.58 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Sat, 23 May 2020 18:01:59 -0700 (PDT) Date: Sat, 23 May 2020 18:01:36 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Konstantin Khlebnikov cc: Andrew Morton , Vlastimil Babka , David Rientjes , Hugh Dickins , "Kirill A. Shutemov" , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount() In-Reply-To: <158937872515.474360.5066096871639561424.stgit@buzz> Message-ID: References: <158937872515.474360.5066096871639561424.stgit@buzz> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 13 May 2020, Konstantin Khlebnikov wrote: > Function isolate_migratepages_block() runs some checks out of lru_lock > when choose pages for migration. After checking PageLRU() it checks extra > page references by comparing page_count() and page_mapcount(). Between > these two checks page could be removed from lru, freed and taken by slab. > > As a result this race triggers VM_BUG_ON(PageSlab()) in page_mapcount(). > Race window is tiny. For certain workload this happens around once a year. Around once a year, that was my guess too. I have no record of us ever hitting this, but yes it could happen when you have CONFIG_DEBUG_VM=y (which I too like to run with, but would not recommend for users). > > > page:ffffea0105ca9380 count:1 mapcount:0 mapping:ffff88ff7712c180 index:0x0 compound_mapcount: 0 > flags: 0x500000000008100(slab|head) > raw: 0500000000008100 dead000000000100 dead000000000200 ffff88ff7712c180 > raw: 0000000000000000 0000000080200020 00000001ffffffff 0000000000000000 > page dumped because: VM_BUG_ON_PAGE(PageSlab(page)) > ------------[ cut here ]------------ > kernel BUG at ./include/linux/mm.h:628! > invalid opcode: 0000 [#1] SMP NOPTI > CPU: 77 PID: 504 Comm: kcompactd1 Tainted: G W 4.19.109-27 #1 > Hardware name: Yandex T175-N41-Y3N/MY81-EX0-Y3N, BIOS R05 06/20/2019 > RIP: 0010:isolate_migratepages_block+0x986/0x9b0 > > > To fix just opencode page_mapcount() in racy check for 0-order case and > recheck carefully under lru_lock when page cannot escape from lru. > > Also add checking extra references for file pages and swap cache. > > Signed-off-by: Konstantin Khlebnikov > Fixes: 119d6d59dcc0 ("mm, compaction: avoid isolating pinned pages") Not really, that commit was correct at the time it went in. > Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()") Exactly, that commit was well-intentioned, but did not allow for this (admittedly very exceptional) usage. How many developers actually make the mistake of applying page_mapcount() to their slab pages? None, I expect. That VM_BUG_ON_PAGE() is there for documentation, and could just be replaced by a comment - and Linus would be happy with that. > --- > mm/compaction.c | 17 +++++++++++++---- > 1 file changed, 13 insertions(+), 4 deletions(-) > > diff --git a/mm/compaction.c b/mm/compaction.c > index 46f0fcc93081..91bb87fd9420 100644 > --- a/mm/compaction.c > +++ b/mm/compaction.c > @@ -935,12 +935,16 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, > } > > /* > - * Migration will fail if an anonymous page is pinned in memory, > + * Migration will fail if an page is pinned in memory, > * so avoid taking lru_lock and isolating it unnecessarily in an > - * admittedly racy check. > + * admittedly racy check simplest case for 0-order pages. > + * > + * Open code page_mapcount() to avoid VM_BUG_ON(PageSlab(page)). But open coding page_mapcount() is not all that you did. You have (understandably) chosen to avoid calling page_mapping(page), but... > + * Page could have extra reference from mapping or swap cache. > */ > - if (!page_mapping(page) && > - page_count(page) > page_mapcount(page)) > + if (!PageCompound(page) && > + page_count(page) > atomic_read(&page->_mapcount) + 1 + > + (!PageAnon(page) || PageSwapCache(page))) > goto isolate_fail; Isn't that test going to send all the file cache pages with buffer heads in page->private, off to isolate_fail when they're actually great candidates for migration? Given that the actual bug spotted was with the VM_BUG_ON_PAGE(PageSlab), and nobody has reported any crash from the use of page_mapping() there (and we only need the test to be right most of the time: all of this knowingly racy, as you explain in other mail): I'd go for just replacing the VM_BUG_ON_PAGE in page_mapcount() by a comment about this case. But if you think developers are really in danger of coding page_mapcount() on their slab pages, then you could add a _page_mapcount() to linux/mm.h, which omits the VM_BUG_ON_PAGE, for use here only. Then we wouldn't have to think so hard about the counting above! > > /* > @@ -975,6 +979,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, > low_pfn += compound_nr(page) - 1; > goto isolate_fail; > } > + > + /* Recheck page extra references under lock */ > + if (page_count(page) > page_mapcount(page) + > + (!PageAnon(page) || PageSwapCache(page))) > + goto isolate_fail; Well, that lru_lock (and the intervening PageLRU check after getting it) may restrict PageAnon and PageSwapCache transitions to some extent, but it certainly has no effect on page_count and page_mapcount: so I think such an additional check here is rather superfluous, and we should just rely on the final checks in migrate_page_move_mapping(), as before. > } > > lruvec = mem_cgroup_page_lruvec(page, pgdat);