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Date:	Fri, 10 Oct 2014 11:20:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alex Thorlton <athorlton@....com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>, Bob Liu <lliubbo@...il.com>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org
Subject: Re: [BUG] mm, thp: khugepaged can't allocate on requested node when
 confined to a cpuset

On Wed, Oct 08, 2014 at 02:10:50PM -0500, Alex Thorlton wrote:

> Is this particular bug a known issue?

Its not unexpected for me.

> I've been trying to come up with
> a simple way to fix the bug, but it's a bit difficult since we no longer
> have a way to trace back to the task_struct that we're collapsing for
> once we've reached get_page_from_freelist.  I'm wondering if we might
> want to make the cpuset check higher up in the call-chain and then pass
> that nodemask down instead of sending a NULL nodemask, as we end up
> doing in many (most?) situations.  I can think of several problems with
> that approach as well, but it's all I've come up with so far.
> 
> The obvious workaround is to not isolate khugepaged to a cpuset, but
> since we're allowed to do so, I think the thread should probably behave
> appropriately when pinned to a cpuset.
> 
> Any input on this issue is greatly appreciated.  Thanks, guys!

So for the numa thing we do everything from the affected tasks context.
There was a lot of arguments early on that that could never really work,
but here we are.

Should we convert khugepaged to the same? Drive the whole thing from
task_work? That would make this issue naturally go away.
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