linux-kernel - Re: [PATCH 00/31] numa/core patches

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Date:	Fri, 2 Nov 2012 16:06:44 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Zhouping Liu <zliu@...hat.com>
cc:	Johannes Weiner <hannes@...xchg.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...nel.org>, CAI Qian <caiqian@...hat.com>
Subject: Re: [PATCH 00/31] numa/core patches

On Fri, 2 Nov 2012, Zhouping Liu wrote:
> On 11/01/2012 09:41 PM, Hugh Dickins wrote:
> > 
> > Here's a patch fixing and tidying up that and a few other things there.
> > But I'm not signing it off yet, partly because I've barely tested it
> > (quite probably I didn't even have any numa pmd migration happening
> > at all), and partly because just a moment ago I ran across this
> > instructive comment in __collapse_huge_page_isolate():
> > 	/* cannot use mapcount: can't collapse if there's a gup pin */
> > 	if (page_count(page) != 1) {
> > 
> > Hmm, yes, below I've added the page_mapcount() check I proposed to
> > do_huge_pmd_numa_page(), but is even that safe enough?  Do we actually
> > need a page_count() check (for 2?) to guard against get_user_pages()?
> > I suspect we do, but then do we have enough locking to stabilize such
> > a check?  Probably, but...
> > 
> > This will take more time, and I doubt get_user_pages() is an issue in
> > your testing, so please would you try the patch below, to see if it
> > does fix the BUGs you are seeing?  Thanks a lot.
> 
> Hugh, I have tested the patch for 5 more hours,
> the issue can't be reproduced again,
> so I think it has fixed the issue, thank you :)

Thanks a lot for testing and reporting back, that's good news.

However, I've meanwhile become convinced that more fixes are needed here,
to be safe against get_user_pages() (including get_user_pages_fast());
to get the Mlocked count right; and to recover correctly when !pmd_same
with an Unevictable page.

Won't now have time to update the patch today,
but these additional fixes shouldn't hold up your testing.

Hugh
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