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Date: Wed, 14 Sep 2016 11:05:01 +0200
From: Michal Hocko <mhocko@suse.cz>
Subject: Re: [PATCH] mm: fix oom work when memory is under pressure
Message-ID: <20160914090500.GC1612@dhcp22.suse.cz>
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To: zhong jiang <zhongjiang@huawei.com>
Cc: akpm@linux-foundation.org, vbabka@suse.cz, rientjes@google.com, linux-mm@kvack.org, Xishi Qiu <qiuxishi@huawei.com>, Hanjun Guo <guohanjun@huawei.com>

On Wed 14-09-16 16:50:48, zhong jiang wrote:
> On 2016/9/14 16:42, Michal Hocko wrote:
> > [Let's CC Hugh]
> >
> > On Wed 14-09-16 15:13:50, zhong jiang wrote:
> > [...]
> >>   hi, Michal
> >>
> >>   Recently, I hit the same issue when run a OOM case of the LTP and ksm enable.
> >>  
> >> [  601.937145] Call trace:
> >> [  601.939600] [<ffffffc000086a88>] __switch_to+0x74/0x8c
> >> [  601.944760] [<ffffffc000a1bae0>] __schedule+0x23c/0x7bc
> >> [  601.950007] [<ffffffc000a1c09c>] schedule+0x3c/0x94
> >> [  601.954907] [<ffffffc000a1eb84>] rwsem_down_write_failed+0x214/0x350
> >> [  601.961289] [<ffffffc000a1e32c>] down_write+0x64/0x80
> >> [  601.966363] [<ffffffc00021f794>] __ksm_exit+0x90/0x19c
> >> [  601.971523] [<ffffffc0000be650>] mmput+0x118/0x11c
> >> [  601.976335] [<ffffffc0000c3ec4>] do_exit+0x2dc/0xa74
> >> [  601.981321] [<ffffffc0000c46f8>] do_group_exit+0x4c/0xe4
> >> [  601.986656] [<ffffffc0000d0f34>] get_signal+0x444/0x5e0
> >> [  601.991904] [<ffffffc000089fcc>] do_signal+0x1d8/0x450
> >> [  601.997065] [<ffffffc00008a35c>] do_notify_resume+0x70/0x78
> > So this is a hung task triggering because the exiting task cannot get
> > the mmap sem for write because the ksmd holds it for read while
> > allocating memory which just takes ages to complete, right?
>   Yes
> >> The root case is that ksmd hold the read lock. and the lock is not released.
> >>  scan_get_next_rmap_item
> >>          down_read
> >>                    get_next_rmap_item
> >>                              alloc_rmap_item     #ksmd will loop permanently.
> >>
> >> How do you see this kind of situation ? or  let the issue alone.
> > I am not familiar with the ksmd code so it is hard for me to judge but
> > one thing to do would be __GFP_NORETRY which would force a bail out from
> > the allocation rather than looping for ever. A quick look tells me that
> > the allocation failure here is quite easy to handle. There might be
> > others...
> >
>   by adding my patch , The question is fixed.  They are same issue.

No, it's not, as I've alreade mentioned before.

-- 
Michal Hocko
SUSE Labs

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