linux-kernel - Re: [PATCH -alternative] mm: hugetlbfs: Close race during teardown of hugetlbfs shared page tables V2 (resend)

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Date:	Fri, 20 Jul 2012 15:51:21 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Linux-MM <linux-mm@...ck.org>, Hugh Dickins <hughd@...gle.com>,
	David Gibson <david@...son.dropbear.id.au>,
	Ken Chen <kenchen@...gle.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -alternative] mm: hugetlbfs: Close race during teardown
 of hugetlbfs shared page tables V2 (resend)

On Fri, Jul 20, 2012 at 04:36:35PM +0200, Michal Hocko wrote:
> And here is my attempt for the fix (Hugh mentioned something similar
> earlier but he suggested using special flags in ptes or VMAs). I still
> owe doc. update and it hasn't been tested with too many configs and I
> could missed some definition updates.
> I also think that changelog could be much better, I will add (steal) the
> full bug description if people think that this way is worth going rather
> than the one suggested by Mel.
> To be honest I am not quite happy how I had to pollute generic mm code with
> something that is specific to a single architecture.
> Mel hammered it with the test case and it survived.

Tested-by: Mel Gorman <mgorman@...e.de>

This approach looks more or less like what I was expecting. I like that
the trick was applied to the page table page instead of using PTE tricks
or by bodging it with a VMA flag like I was thinking so kudos for that. I
also prefer this approach to trying to free the page tables on or near
huge_pmd_unshare()

In general I think this patch would execute better than mine because it is
far less heavy-handed but I share your concern that it changes the core MM
quite a bit for a corner case that only one architecture cares about. I am
completely biased of course, but I still prefer my patch because other than
an API change it keeps the bulk of the madness in arch/x86/mm/hugetlbpage.c
. I am also not concerned with the scalability of how quickly we can setup
page table sharing.

Hugh, I'm afraid you get to choose :)

-- 
Mel Gorman
SUSE Labs
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