linux-kernel - Re: [PATCHv2] Add per-process flag to control thp

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Date:	Mon, 05 Aug 2013 11:09:20 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Alex Thorlton <athorlton@....com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Dave Jones <davej@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Kees Cook <keescook@...omium.org>, Robin Holt <holt@....com>
Subject: Re: [PATCHv2] Add per-process flag to control thp

On 08/03/2013 01:01 PM, Oleg Nesterov wrote:

>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +	p->thp_disabled = current->thp_disabled;
>> +#endif
>
> Unneeded. It will be copied by dup_task_struct() automagically.
>
> But I simply can't understand why this flag is per-thread. It should be
> mm flag, no?

It has to be per-mm for another reason, too.

Think about what were to happen if one process were ptracing
a process with the thp_disabled flag, and ended up causing a
new anonymous page to be faulted in.

With the thp_disabled flag in the task, get_user_pages would
end up not seeing the flag, and the task could get a transparent
huge page.

With the thp_disabled flag in the mm, it would be possible for
get_user_pages to easily find the flag, and do the right thing.

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