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Date:	Thu, 2 Jun 2016 21:08:57 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>, linux-mm@...ck.org,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [linux-next: Tree for Jun 1] __khugepaged_exit
 rwsem_down_write_failed lockup

Hello Michal,

On (06/02/16 11:21), Michal Hocko wrote:
[..]
> > [ 2856.323052] INFO: task cc1:4582 blocked for more than 21 seconds.
> > [ 2856.323055]       Not tainted 4.7.0-rc1-next-20160601-dbg-00012-g52c180e-dirty #453
> > [ 2856.323056] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [ 2856.323059] cc1             D ffff880057e9fd78     0  4582   4575 0x00000000
> > [ 2856.323062]  ffff880057e9fd78 ffff880057e08000 ffff880057e9fd90 ffff880057ea0000
> > [ 2856.323065]  ffff88005dc3dc68 ffffffff00000001 ffff880057e09500 ffff88005dc3dc80
> > [ 2856.323067]  ffff880057e9fd90 ffffffff81441e33 ffff88005dc3dc68 ffff880057e9fe00
> > [ 2856.323068] Call Trace:
> > [ 2856.323074]  [<ffffffff81441e33>] schedule+0x83/0x98
> > [ 2856.323077]  [<ffffffff81443d9b>] rwsem_down_write_failed+0x18e/0x1d3
> > [ 2856.323080]  [<ffffffff810a87cf>] ? unlock_page+0x2b/0x2d
> > [ 2856.323083]  [<ffffffff811bdb77>] call_rwsem_down_write_failed+0x17/0x30
> > [ 2856.323084]  [<ffffffff811bdb77>] ? call_rwsem_down_write_failed+0x17/0x30
> > [ 2856.323086]  [<ffffffff81443630>] down_write+0x1f/0x2e
> > [ 2856.323089]  [<ffffffff810ea4f3>] __khugepaged_exit+0x104/0x11a
> > [ 2856.323091]  [<ffffffff8103702a>] mmput+0x29/0xc5
> > [ 2856.323093]  [<ffffffff8103bbd8>] do_exit+0x34c/0x894
> > [ 2856.323095]  [<ffffffff8102f9e0>] ? __do_page_fault+0x2f7/0x399
> > [ 2856.323097]  [<ffffffff8103c188>] do_group_exit+0x3c/0x98
> > [ 2856.323099]  [<ffffffff8103c1f3>] SyS_exit_group+0xf/0xf
> > [ 2856.323101]  [<ffffffff81444cdb>] entry_SYSCALL_64_fastpath+0x13/0x8f
> 
> down_write in the exit path is certainly not nice. It is hard to tell
> who is blocking the mmap_sem but it is clear that __khugepaged_exit
> waits for the khugepaged to release its mmap_sem. Do you hapen to have a
> trace of khugepaged? Note that the lock holder might be another writer
> which just hasn't pinned mm_users so khugepaged might be blocked on read
> lock as well. Or khugepaged might be just stuck somewhere...

sorry, no. this is all I have. the kernel was compiled with almost no
debugging functionality enabled (no lockdep, no lock debug, nothing)
for zram performance testing purposes.

I'll try to reproduce the problem; and give your patch some testing.
thanks.

	-ss

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