Re: [PATCH v2 1/2] SUNRPC: Fix memory reclaim deadlocks in rpciod

[Junxiao Bi <junxiao.bi@oracle.com>]

On 08/26/2014 02:21 PM, NeilBrown wrote:
> On Tue, 26 Aug 2014 13:43:47 +0800 Junxiao Bi <junxiao.bi@oracle.com> wrote:
> 
>> On 08/25/2014 02:48 PM, NeilBrown wrote:
>>> On Fri, 22 Aug 2014 18:49:31 -0400 Trond Myklebust
>>> <trond.myklebust@primarydata.com> 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?

Thanks,
Junxiao.

> 
> NeilBrown
>