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Message-ID: <20130424152043.GP2018@cmpxchg.org> Date: Wed, 24 Apr 2013 11:20:43 -0400 From: Johannes Weiner <hannes@...xchg.org> To: Michal Hocko <mhocko@...e.cz> Cc: Heiko Carstens <heiko.carstens@...ibm.com>, Zhouping Liu <zliu@...hat.com>, Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Glauber Costa <glommer@...allels.com>, linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>, caiqian <caiqian@...hat.com>, Caspar Zhang <czhang@...hat.com>, Martin Schwidefsky <schwidefsky@...ibm.com> Subject: Re: [v3.9-rc8]: kernel BUG at mm/memcontrol.c:3994! (was: Re: [BUG][s390x] mm: system crashed) On Wed, Apr 24, 2013 at 03:18:51PM +0200, Michal Hocko wrote: > On Wed 24-04-13 12:42:55, Heiko Carstens wrote: > > On Thu, Apr 18, 2013 at 09:13:03AM +0200, Heiko Carstens wrote: > > > Ok, thanks for verifying! I'll look into it; hopefully I can reproduce it > > > here as well. > > > > That seems to be a common code bug. I can easily trigger the VM_BUG_ON() > > below (when I force the system to swap): > > > > [ 48.347963] ------------[ cut here ]------------ > > [ 48.347972] kernel BUG at mm/memcontrol.c:3994! > > [ 48.348012] illegal operation: 0001 [#1] SMP > > [ 48.348015] Modules linked in: > > [ 48.348017] CPU: 1 Not tainted 3.9.0-rc8+ #38 > > [ 48.348020] Process mmap2 (pid: 635, task: 0000000029476100, ksp: 000000002e91b938) > > [ 48.348022] Krnl PSW : 0704f00180000000 000000000026552c (__mem_cgroup_uncharge_common+0x2c4/0x33c) > > [ 48.348032] R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:3 PM:0 EA:3 > > Krnl GPRS: 0000000000000008 0000000000000009 000003d1002a9200 0000000000000000 > > [ 48.348039] 0000000000000000 00000000006812d8 000003ffdf339000 00000000321a6f98 > > [ 48.348043] 000003fffce11000 0000000000000000 0000000000000001 000003d1002a9200 > > [ 48.348046] 0000000000000001 0000000000681b88 000000002e91bc18 000000002e91bbd0 > > [ 48.348057] Krnl Code: 000000000026551e: c0e5fffaa2a1 brasl %r14,1b9a60 > > 0000000000265524: a7f4ff7d brc 15,26541e > > #0000000000265528: a7f40001 brc 15,26552a > > >000000000026552c: e3c0b8200124 stg %r12,6176(%r11) > > 0000000000265532: a7f4ff57 brc 15,2653e0 > > 0000000000265536: e310b8280104 lg %r1,6184(%r11) > > 000000000026553c: a71b0001 aghi %r1,1 > > 0000000000265540: e310b8280124 stg %r1,6184(%r11) > > [ 48.348099] Call Trace: > > [ 48.348100] ([<000003d1002a91c0>] 0x3d1002a91c0) > > [ 48.348102] [<00000000002404aa>] page_remove_rmap+0xf2/0x16c > > [ 48.348106] [<0000000000232dc8>] unmap_single_vma+0x494/0x7d8 > > [ 48.348107] [<0000000000233ac0>] unmap_vmas+0x50/0x74 > > [ 48.348109] [<00000000002396ec>] unmap_region+0x9c/0x110 > > [ 48.348110] [<000000000023bd18>] do_munmap+0x284/0x470 > > [ 48.348111] [<000000000023bf56>] vm_munmap+0x52/0x70 > > [ 48.348113] [<000000000023cf32>] SyS_munmap+0x3a/0x4c > > [ 48.348114] [<0000000000665e14>] sysc_noemu+0x22/0x28 > > [ 48.348118] [<000003fffcf187b2>] 0x3fffcf187b2 > > [ 48.348119] Last Breaking-Event-Address: > > [ 48.348120] [<0000000000265528>] __mem_cgroup_uncharge_common+0x2c0/0x33c > > > > Looking at the code, the code flow is: > > > > page_remove_rmap() -> mem_cgroup_uncharge_page() -> __mem_cgroup_uncharge_common() > > > > Note that in mem_cgroup_uncharge_page() the page in question passed the check: > > > > [...] > > if (PageSwapCache(page)) > > return; > > [...] > > > > and just a couple of instructions later the VM_BUG_ON() within > > __mem_cgroup_uncharge_common() triggers: > > > > [...] > > if (mem_cgroup_disabled()) > > return NULL; > > > > VM_BUG_ON(PageSwapCache(page)); > > [...] > > > > Which means that another cpu changed the pageflags concurrently. In fact, > > looking at the dump a different cpu is indeed busy with running kswapd. > > Hmm, maybe I am missing something but it really looks like we can race > here. Reclaim path takes the page lock while zap_pte takes page table > lock so nothing prevents them from racing here: > shrink_page_list zap_pte_range > trylock_page pte_offset_map_lock > add_to_swap page_remove_rmap > /* Page can be still mapped */ > add_to_swap_cache atomic_add_negative(_mapcount) > __add_to_swap_cache mem_cgroup_uncharge_page > (PageSwapCache(page)) && return > SetPageSwapCache > __mem_cgroup_uncharge_common > VM_BUG_ON(PageSwapCache(page)) > > Maybe not many people run with CONFIG_DEBUG_VM enabled these days so we > do not this more often (even me testing configs are not consistent in > that regards and only few have it on). The only thing that changed in > this area recently is 0c59b89c which made the test VM_BUG_ON rather then > simple return in 3.6 > And maybe the BUG_ON is too harsh as CgroupUsed should guarantee that > the uncharge will eventually go away. What do you think Johannes? Interesting. We need to ensure there is ordering between setting PG_swapcache and installing swap entries because I think we are the only ones looking at PG_swapcache without the page lock held. So we don't have a safe way to check for PG_swapcache but if we get it wrong, we may steal an uncharge that uncharge_swapcache() should be doing instead and that means we mess up the swap statistics accounting. So how can we, without holding the page lock, either safely back off from a page in swapcache or make sure we do the swap statistics accounting when uncharging a swapcache page from the final unmap? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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