linux-kernel - Re: [patch for-3.7] mm, mempolicy: fix printing stack contents in numa

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Date:	Wed, 17 Oct 2012 04:49:02 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	bhutchings@...arflare.com,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch for-3.7] mm, mempolicy: fix printing stack contents in numa_maps

On Wed, Oct 17, 2012 at 1:42 AM, Kamezawa Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> (2012/10/17 14:24), David Rientjes wrote:
>>
>> On Wed, 17 Oct 2012, Dave Jones wrote:
>>
>>> BUG: sleeping function called from invalid context at kernel/mutex.c:269
>>> in_atomic(): 1, irqs_disabled(): 0, pid: 8558, name: trinity-child2
>>> 3 locks on stack by trinity-child2/8558:
>>>   #0: held:     (&p->lock){+.+.+.}, instance: ffff88010c9a00b0, at:
>>> [<ffffffff8120cd1f>] seq_lseek+0x3f/0x120
>>>   #1: held:     (&mm->mmap_sem){++++++}, instance: ffff88013956f7c8, at:
>>> [<ffffffff81254437>] m_start+0xa7/0x190
>>>   #2: held:     (&(&p->alloc_lock)->rlock){+.+...}, instance:
>>> ffff88011fc64f30, at: [<ffffffff81254f8f>] show_numa_map+0x14f/0x610
>>> Pid: 8558, comm: trinity-child2 Not tainted 3.7.0-rc1+ #32
>>> Call Trace:
>>>   [<ffffffff810ae4ec>] __might_sleep+0x14c/0x200
>>>   [<ffffffff816bdf4e>] mutex_lock_nested+0x2e/0x50
>>>   [<ffffffff811c43a3>] mpol_shared_policy_lookup+0x33/0x90
>>>   [<ffffffff8118d5c3>] shmem_get_policy+0x33/0x40
>>>   [<ffffffff811c31fa>] get_vma_policy+0x3a/0x90
>>>   [<ffffffff81254fa3>] show_numa_map+0x163/0x610
>>>   [<ffffffff81255b10>] ? pid_maps_open+0x20/0x20
>>>   [<ffffffff81255980>] ? pagemap_hugetlb_range+0xf0/0xf0
>>>   [<ffffffff81255483>] show_pid_numa_map+0x13/0x20
>>>   [<ffffffff8120c902>] traverse+0xf2/0x230
>>>   [<ffffffff8120cd8b>] seq_lseek+0xab/0x120
>>>   [<ffffffff811e6c0b>] sys_lseek+0x7b/0xb0
>>>   [<ffffffff816ca088>] tracesys+0xe1/0xe6
>>>
>>
>> Hmm, looks like we need to change the refcount semantics entirely.  We'll
>> need to make get_vma_policy() always take a reference and then drop it
>> accordingly.  This work sif get_vma_policy() can grab a reference while
>> holding task_lock() for the task policy fallback case.
>>
>> Comments on this approach?
>
>
> I think this refcounting is better than using task_lock().

I don't think so. get_vma_policy() is used from fast path. In other
words, number of
atomic ops is sensible for allocation performance. Instead, I'd like
to use spinlock
for shared mempolicy instead of mutex.
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