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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id x7sm410802ooj.16.2020.06.01.21.05.38 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Mon, 01 Jun 2020 21:05:39 -0700 (PDT) Date: Mon, 1 Jun 2020 21:05:25 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Andrew Morton cc: Konstantin Khlebnikov , Vlastimil Babka , David Rientjes , Hugh Dickins , "Kirill A. Shutemov" , Alex Shi , Johannes Weiner , 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: 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 X-Rspamd-Queue-Id: B7DA037604 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, 23 May 2020, Hugh Dickins wrote: > 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); Andrew, I've noticed that this buggy mm-compaction-avoid-vm_bug_onpageslab-in-page_mapcount.patch was still in Friday's mmotm 2020-05-29-16-09, despite its replacement 6988f31d558a ("mm: remove VM_BUG_ON(PageSlab()) from page_mapcount()") getting into 5.7, thanks to your "incoming" to Linus on that day. Please be sure to remove this patch to mm/compaction.c from your tree and queue to Linus for 5.8: it imposes an unintended and significant limitation on the current behavior of compaction. (And in some loads, some of that additional limitation may actually be beneficial: but if so, must be argued separately, not as page_mapcount BUG avoidance). Cc'ing Alex Shi, because I noticed this when trying his v11 per-memcg lru_lock series (which appears to be a big improvement over earlier versions, thanks in particular to Johannes's memcg swap simplifications); and Alex's 12/16 makes a change on top of Konstantin's latter check, which will now just be reverted. I'm not yet confident in Alex's isolate_migratepages_block(), in part because this muddle. Thanks, Hugh