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Date: Tue, 30 Apr 2013 13:27:11 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Hugh Dickins <hughd@...gle.com>
Cc: Michal Hocko <mhocko@...e.cz>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Zhouping Liu <zliu@...hat.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>,
Lingzhu Xiang <lxiang@...hat.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 08:50:01PM -0700, Hugh Dickins wrote:
> On Wed, 24 Apr 2013, Johannes Weiner wrote:
> > 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?
>
> Awkward.
>
> I agree that the actual memcg uncharging should be okay, but the memsw
> swap stats will go wrong (doesn't matter toooo much), and mem_cgroup_put
> get missed (leaking a struct mem_cgroup).
Ok, so I just went over this again. For the swapout path the memsw
uncharge is deferred, but if we "steal" this uncharge from the swap
code, we actually do uncharge memsw in mem_cgroup_do_uncharge(), so we
may prematurely unaccount the swap page, but we never leak a charge.
Good.
Because of this stealing, we also don't do the following:
if (do_swap_account && ctype == MEM_CGROUP_CHARGE_TYPE_SWAPOUT) {
mem_cgroup_swap_statistics(memcg, true);
mem_cgroup_get(memcg);
}
I.e. it does not matter that mem_cgroup_uncharge_swap() doesn't do the
put, we are also not doing the get. We should not leak references.
So the only thing that I can see go wrong is that we may have a
swapped out page that is not charged to memsw and not accounted as
MEM_CGROUP_STAT_SWAP. But I don't know how likely that is, because we
check for PG_swapcache in this uncharge path after the last pte is
torn down, so even though the page is put on swap cache, it probably
won't be swapped. It would require that the PG_swapcache setting
would become visible only after the page has been added to the swap
cache AND rmap has established at least one swap pte for us to
uncharge a page that actually continues to be used. And that's a bit
of a stretch, I think.
Did I miss something? If not, I'll just send a patch that removes the
VM_BUG_ON() and adds a comment describing the scenarios and a note
that we may want to fix this in the future.
Thanks!
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