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

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Message-ID: <CAHGf_=ojnEA6FG2eE_P6oAO-WtybAbTF8xNUMWb4geF4PXQbhA@mail.gmail.com>
Date:	Wed, 17 Oct 2012 17:05:37 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.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 3:50 PM, David Rientjes <rientjes@...gle.com> wrote:
> On Wed, 17 Oct 2012, KOSAKI Motohiro wrote:
>
>> > 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.
>
> There are enhancements that we can make with refcounting: for instance, we
> may want to avoid doing it in the super-fast path when the policy is
> default_policy and then just do
>
>         if (mpol != &default_policy)
>                 mpol_put(mpol);
>
>> Instead, I'd like
>> to use spinlock
>> for shared mempolicy instead of mutex.
>>
>
> Um, this was just changed to a mutex last week in commit b22d127a39dd
> ("mempolicy: fix a race in shared_policy_replace()") so that sp_alloc()
> can be done with GFP_KERNEL, so I didn't consider reverting that behavior.
> Are you nacking that patch, which you acked, now?

Yes, sadly. /proc usage is a corner case issue. It's not worth to
strike main path.
see commit 52cd3b0740 and around patches. That explain why we avoided your
approach.
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