Re: kernel BUG in munlock_vma_pages_range

From: Vlastimil Babka
Date: Fri Mar 21 2014 - 05:02:24 EST

On 03/21/2014 02:53 AM, Sasha Levin wrote:
On 12/13/2013 04:08 AM, Vlastimil Babka wrote:
On 12/13/2013 09:49 AM, Bob Liu wrote:
On 12/13/2013 05:05 AM, Sasha Levin wrote:
On 12/12/2013 07:41 AM, Vlastimil Babka wrote:
On 12/12/2013 06:03 AM, Bob Liu wrote:

On 12/12/2013 11:16 AM, Sasha Levin wrote:
On 12/11/2013 05:59 PM, Vlastimil Babka wrote:
On 12/09/2013 09:26 PM, Sasha Levin wrote:
On 12/09/2013 12:12 PM, Vlastimil Babka wrote:
On 12/09/2013 06:05 PM, Sasha Levin wrote:
On 12/09/2013 04:34 AM, Vlastimil Babka wrote:
Hello, I will look at it, thanks.
Do you have specific reproduction instructions?

Not really, the fuzzer hit it once and I've been unable to trigger
it again. Looking at
the piece of code involved it might have had something to do with
hugetlbfs, so I'll crank
up testing on that part.

Thanks. Do you have trinity log and the .config file? I'm currently
unable to even boot linux-next
with my config/setup due to a GPF.
Looking at code I wouldn't expect that it could encounter a tail
page, without first encountering a
head page and skipping the whole huge page. At least in THP case, as
TLB pages should be split when
a vma is split. As for hugetlbfs, it should be skipped for
mlock/munlock operations completely. One
of these assumptions is probably failing here...

If it helps, I've added a dump_page() in case we hit a tail page
there and got:

[ 980.172299] page:ffffea003e5e8040 count:0 mapcount:1
mapping: (null) index:0
[ 980.173412] page flags: 0x2fffff80008000(tail)

I can also add anything else in there to get other debug output if
you think of something else useful.

Please try the following. Thanks in advance.

[ 428.499889] page:ffffea003e5c0040 count:0 mapcount:4
mapping: (null) index:0x0
[ 428.499889] page flags: 0x2fffff80008000(tail)
[ 428.499889] start=140117131923456 pfn=16347137
orig_start=140117130543104 page_increm
=1 vm_start=140117130543104 vm_end=140117134688256 vm_flags=135266419
[ 428.499889] first_page pfn=16347136
[ 428.499889] page:ffffea003e5c0000 count:204 mapcount:44
mapping:ffff880fb5c466c1 inde
[ 428.499889] page flags:

From this print, it looks like the page is still a huge page.
One situation I guess is a huge page which isn't PageMlocked and passed
to munlock_vma_page(). I'm not sure whether this will happen.

Yes that's quite likely the case. It's not illegal to happen I would say.

Please take a try this patch.

I've made a simpler version that does away with the ugly page_mask
thing completely.
Please try that as well. Thanks.

Also when working on this I think I found another potential but much
rare problem
when munlock_vma_page races with a THP split. That would however
manifest such that
part of the former tail pages would stay PageMlocked. But that still
needs more thought.
The bug at hand should however be fixed by this patch.

Yup, this patch seems to fix the issue previously reported.

However, I'll piggyback another thing that popped up now that the vm
could run for a while which
also seems to be caused by the original patch. It looks like a pretty
straightforward deadlock, but

Sigh, put one down, patch it around... :)

Looks like put_page() in __munlock_pagevec() need to get the
zone->lru_lock which is already held when entering __munlock_pagevec().

I've come to the same conclusion, however:

How about fix like this?

That unfortunately removes most of the purpose of this function which was to avoid repeated locking.

Please try this patch.

It seems that this one is back, not exactly sure why yet:

Hm? You reported this already 6 days ago in this thread:

Then I sent a debug patch and from your results concluded something is probably wrong with the vma and suspected vma caching:

However next-20140320 means that the latest fixes to vma caching were applied, so I have no idea now. But reapplying the debug patch shouldn't hurt, maybe it will yield something else/more useful this time.


[ 2857.034927] kernel BUG at include/linux/page-flags.h:415!
[ 2857.035576] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 2857.036702] Dumping ftrace buffer:
[ 2857.037447] (ftrace buffer empty)
[ 2857.037937] Modules linked in:
[ 2857.038379] CPU: 25 PID: 21381 Comm: trinity-c61 Tainted: G W 3.14.0-rc7-next-20140320-sasha-00015-gd752393-dirty #261
[ 2857.039854] task: ffff88080f91b000 ti: ffff8807fd106000 task.ti: ffff8807fd106000
[ 2857.040328] RIP: 0010:[<ffffffff8129dc93>] [<ffffffff8129dc93>] munlock_vma_pages_range+0x93/0x1d0
[ 2857.040328] RSP: 0000:ffff8807fd107e08 EFLAGS: 00010246
[ 2857.040328] RAX: ffff88052c955360 RBX: 0000000041b36000 RCX: 000000000000009f
[ 2857.040328] RDX: 0000000000000000 RSI: ffff88080f91bcf0 RDI: 0000000004fd5360
[ 2857.040328] RBP: ffff8807fd107ec8 R08: 0000000000000001 R09: 0000000000000000
[ 2857.040328] R10: 0000000000000001 R11: 0000000000000001 R12: ffffea0013f54d80
[ 2857.040328] R13: ffff88068083c200 R14: 0000000041b37000 R15: ffff8807fd107e94
[ 2857.040328] FS: 00007fcd4bd02700(0000) GS:ffff8806acc00000(0000) knlGS:0000000000000000
[ 2857.040328] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 2857.040328] CR2: 00000000027405a8 CR3: 0000000804ad4000 CR4: 00000000000006a0
[ 2857.040328] DR0: 0000000000698000 DR1: 0000000000698000 DR2: 0000000000000000
[ 2857.040328] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
[ 2857.040328] Stack:
[ 2857.040328] 0000000000000000 0000000000000000 00018807fd107e38 0000000000000000
[ 2857.040328] 0000000000000000 ffff88068083c200 00000000fd107e88 0000000000000000
[ 2857.040328] 00ff8807fd107e58 ffff88052be99b20 ffff8807fd107eb8 ffff88068083c200
[ 2857.040328] Call Trace:
[ 2857.040328] [<ffffffff812a1462>] do_munmap+0x1d2/0x360
[ 2857.040328] [<ffffffff844bce16>] ? down_write+0xa6/0xc0
[ 2857.040328] [<ffffffff812a1636>] ? vm_munmap+0x46/0x80
[ 2857.040328] [<ffffffff812a1644>] vm_munmap+0x54/0x80
[ 2857.040328] [<ffffffff812a169c>] SyS_munmap+0x2c/0x40
[ 2857.040328] [<ffffffff844c9210>] tracesys+0xdd/0xe2
[ 2857.040328] Code: ff 49 89 c4 48 85 c0 0f 84 f3 00 00 00 48 3d 00 f0 ff ff 0f 87 e7 00 00 00 48 8b 00 66 85 c0 79 17 31 f6 4c 89 e7 e8 fd d0 fc ff <0f> 0b 0f 1f 00 eb fe 66 0f 1f 44 00 00 49 8b 04 24 f6 c4 40 74
[ 2857.062774] RIP [<ffffffff8129dc93>] munlock_vma_pages_range+0x93/0x1d0
[ 2857.062774] RSP <ffff8807fd107e08>


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