On 08/26/2014 03:04 PM, NeilBrown wrote:
> On Tue, 26 Aug 2014 14:49:01 +0800 Junxiao Bi <junxiao.bi@xxxxxxxxxx> wrote:
>
>> On 08/26/2014 02:21 PM, NeilBrown wrote:
>>> On Tue, 26 Aug 2014 13:43:47 +0800 Junxiao Bi <junxiao.bi@xxxxxxxxxx> wrote:
>>>
>>>> On 08/25/2014 02:48 PM, NeilBrown wrote:
>>>>> On Fri, 22 Aug 2014 18:49:31 -0400 Trond Myklebust
>>>>> <trond.myklebust@xxxxxxxxxxxxxxx> wrote:
>>>>>
>>>>>> Junxiao Bi reports seeing the following deadlock:
>>>>>>
>>>>>> @ crash> bt 1539
>>>>>> @ PID: 1539 TASK: ffff88178f64a040 CPU: 1 COMMAND: "rpciod/1"
>>>>>> @ #0 [ffff88178f64d2c0] schedule at ffffffff8145833a
>>>>>> @ #1 [ffff88178f64d348] io_schedule at ffffffff8145842c
>>>>>> @ #2 [ffff88178f64d368] sync_page at ffffffff810d8161
>>>>>> @ #3 [ffff88178f64d378] __wait_on_bit at ffffffff8145895b
>>>>>> @ #4 [ffff88178f64d3b8] wait_on_page_bit at ffffffff810d82fe
>>>>>> @ #5 [ffff88178f64d418] wait_on_page_writeback at ffffffff810e2a1a
>>>>>> @ #6 [ffff88178f64d438] shrink_page_list at ffffffff810e34e1
>>>>>> @ #7 [ffff88178f64d588] shrink_list at ffffffff810e3dbe
>>>>>> @ #8 [ffff88178f64d6f8] shrink_zone at ffffffff810e425e
>>>>>> @ #9 [ffff88178f64d7b8] do_try_to_free_pages at ffffffff810e4978
>>>>>> @ #10 [ffff88178f64d828] try_to_free_pages at ffffffff810e4c31
>>>>>> @ #11 [ffff88178f64d8c8] __alloc_pages_nodemask at ffffffff810de370
>>>>>
>>>>> This stack trace (from 2.6.32) cannot happen in mainline, though it took me a
>>>>> while to remember/discover exactly why.
>>>>>
>>>>> try_to_free_pages() creates a 'struct scan_control' with ->target_mem_cgroup
>>>>> set to NULL.
>>>>> shrink_page_list() checks ->target_mem_cgroup using global_reclaim() and if
>>>>> it is NULL, wait_on_page_writeback is *not* called.
>>>>>
>>>>> So we can only hit this deadlock if mem-cgroup limits are imposed on a
>>>>> process which is using NFS - which is quite possible but probably not common.
>>>>>
>>>>> The fact that a dead-lock can happen only when memcg limits are imposed seems
>>>>> very fragile. People aren't going to test that case much so there could well
>>>>> be other deadlock possibilities lurking.
>>>>>
>>>>> Mel: might there be some other way we could get out of this deadlock?
>>>>> Could the wait_on_page_writeback() in shrink_page_list() be made a timed-out
>>>>> wait or something? Any other wait out of this deadlock other than setting
>>>>> PF_MEMALLOC_NOIO everywhere?
>>>>
>>>> Not only the wait_on_page_writeback() cause the deadlock but also the
>>>> next pageout()-> (mapping->a_ops->writepage), Trond's second patch fix
>>>> this. So fix the wait_on_page_writeback is not enough to fix deadlock.
>>>
>>> Shortly before the only place that pageout() is called there is this code:
>>>
>>> if (page_is_file_cache(page) &&
>>> (!current_is_kswapd() ||
>>> !zone_is_reclaim_dirty(zone))) {
>>> .....
>>> goto keep_locked;
>>>
>>>
>>> So pageout() only gets called by kswapd() .... or for swap. swap-over-NFS is
>>> already very cautious about memory allocations, and uses nfs_direct_IO, not
>>> nfs_writepage.
>>>
>>> So nfs_writepage will never get called during direct reclaim. There is no
>>> memory-allocate deadlock risk there.
>> Yes, thanks for explaining this.
>> But is it possible rpciod blocked somewhere by memory allocation using
>> GFP_KERNEL and kswapd is trying to pageout nfs dirty pages and blocked
>> by rpciod?
>
> I don't think so, no.
>
> Only 40% of memory (/proc/sys/vm/dirty_ratio) can be dirty. The direct
> reclaim procedure will eventually find some non-dirty memory it can use.
> If it cannot, and cannot write anything out to swap either, it will
> eventually trigger the OOM killer.
>
> Direct reclaim shouldn't ever block indefinitely. It will sometimes wait for
> a short while (e.g. congestion_wait()) but it should then push on until it
> finds something it can do: free a clean page, write something to swap, or
> kill a memory-hog with the OOM killer.
That makes sense. Thanks.
Junxiao.
>
> NeilBrown
>
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