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

[Mel Gorman <mgorman-IBi9RG/b67k@public.gmane.org>]

On Mon, Aug 25, 2014 at 04:48:52PM +1000, NeilBrown wrote:
> On Fri, 22 Aug 2014 18:49:31 -0400 Trond Myklebust
> <trond.myklebust-7I+n7zu2hftEKMMhf/gKZA@public.gmane.org> 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.
> 

wait_on_page_writeback has a host of other damage associated with it which
is why we don't do it from reclaim any more. If the storage is very slow
then a process can be stalled by unrelated IO to slow storage.  If the
storage is broken and the writeback can never complete then it causes other
issues. That kind of thing.

> 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.
> 

memcgs still can call wait_on_page_writeback and this is known to be a
hand-grenade to the memcg people but I've never heard of them trying to
tackle the problem.

> 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?
> 

I don't have the full thread as it was not cc'd to lkml so I don't know
what circumstances reached this deadlock in the first place. If this is
on 2.6.32 and the deadline cannot happen during reclaim in mainline then
why is mainline being patched?

Do not alter wait_on_page_writeback() to timeout as it will blow
up spectacularly -- swap unuse races, data would not longer be synced
correctly to disk, sync IO would be flaky, stable page writes would be
fired out the window etc.

-- 
Mel Gorman
SUSE Labs
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