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Date: Thu, 4 Sep 2014 12:39:49 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>,
        Kautuk Consul <consul.kautuk@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.cz>, David Rientjes <rientjes@google.com>,
        Ionut Alexa <ionut.m.alexa@gmail.com>,
        Guillaume Morin <guillaume@morinfr.org>, linux-kernel@vger.kernel.org,
        Kirill Tkhai <tkhai@yandex.ru>
Subject: Re: task_numa_fault() && TASK_DEAD
Message-ID: <20140904103949.GA19300@redhat.com>
References: <1408964064-21447-1-git-send-email-consul.kautuk@gmail.com> <20140825155738.GA5944@redhat.com> <20140901153935.GQ27892@worktop.ger.corp.intel.com> <20140901175851.GA15210@redhat.com> <20140901190931.GD5806@worktop.ger.corp.intel.com> <20140902155208.GA28668@redhat.com> <20140902164714.GA17033@redhat.com> <20140903160819.GA7682@redhat.com> <20140904071139.GH3190@worktop.ger.corp.intel.com>
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On 09/04, Peter Zijlstra wrote:
>
> On Wed, Sep 03, 2014 at 06:08:19PM +0200, Oleg Nesterov wrote:
>
> > And a stupid (really, I don't understand this code) question:
> >
> > 	/* for example, ksmd faulting in a user's mm */
> > 	if (!p->mm)
> > 		return;
>
> In general kernel threads have !->mm, and those cannot do the
> accounting. The only way to get here is through get_user_pages() with
> tsk != current and/or mm != current->mm.
>
> > OK, but perhaps it make sense to pass "mm" as another argument and do
> >
> > 	/* ksmd faulting in a user's mm, or debugger, or kthread use_mm() caller */
> > 	if (p->mm != mm)
> > 		return;
> >
> > ?
>
> I'm still somewhat fuzzy in the brain but that doesn't appear to
> actually work, use_mm() explicitly sets ->mm so in that case it would
> match just fine.

Yes, yes, sorry, I meant

	if (p->mm != mm || PF_KTHREAD)
		return;

> That said; I don't think we really need to worry about this. The !->mm
> case is special in that that cannot ever work, the other cases are
> extremely rare and will not skew accounting much if anything.

Sure, this is not bugfix. To me this change looks like a cleanup because
I think that, say, ksmd doesn't really differ from debugger in this case
(ignoring the fact that ->mm == NULL can probably lead to crash), or from
use_mm().

But of course I agree, this is minor.

Oleg.