Re: [PATCH] mm, numa: Fix bad pmd by atomically check for pmd_trans_huge when marking page tables prot_numa

From: Andrew Morton <akpm@linux-foundation.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Zi Yan <zi.yan@cs.rutgers.edu>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Rik van Riel <riel@redhat.com>, Michal Hocko <mhocko@kernel.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm, numa: Fix bad pmd by atomically check for pmd_trans_huge when marking page tables prot_numa
Date: Mon, 10 Apr 2017 15:09:03 -0700	[thread overview]
Message-ID: <20170410150903.f931ceb5475d2d3d8945bb71@linux-foundation.org> (raw)
In-Reply-To: <20170410180714.7yfnxl7qin72jcob@techsingularity.net>

On Mon, 10 Apr 2017 19:07:14 +0100 Mel Gorman <mgorman@techsingularity.net> wrote:

> On Mon, Apr 10, 2017 at 12:49:40PM -0500, Zi Yan wrote:
> > On 10 Apr 2017, at 12:20, Mel Gorman wrote:
> > 
> > > On Mon, Apr 10, 2017 at 11:45:08AM -0500, Zi Yan wrote:
> > >>> While this could be fixed with heavy locking, it's only necessary to
> > >>> make a copy of the PMD on the stack during change_pmd_range and avoid
> > >>> races. A new helper is created for this as the check if quite subtle and the
> > >>> existing similar helpful is not suitable. This passed 154 hours of testing
> > >>> (usually triggers between 20 minutes and 24 hours) without detecting bad
> > >>> PMDs or corruption. A basic test of an autonuma-intensive workload showed
> > >>> no significant change in behaviour.
> > >>>
> > >>> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> > >>> Cc: stable@vger.kernel.org
> > >>
> > >> Does this patch fix the same problem fixed by Kirill's patch here?
> > >> https://lkml.org/lkml/2017/3/2/347
> > >>
> > >
> > > I don't think so. The race I'm concerned with is due to locks not being
> > > held and is in a different path.
> > 
> > I do not agree. Kirill's patch is fixing the same race problem but in
> > zap_pmd_range().
> > 
> > The original autoNUMA code first clears PMD then sets it to protnone entry.
> > pmd_trans_huge() does not return TRUE because it saw cleared PMD, but
> > pmd_none_or_clear_bad() later saw the protnone entry and reported it as bad.
> > Is this the problem you are trying solve?
> > 
> > Kirill's patch will pmdp_invalidate() the PMD entry, which keeps _PAGE_PSE bit,
> > so pmd_trans_huge() will return TRUE. In this case, it also fixes
> > your race problem in change_pmd_range().
> > 
> > Let me know if I miss anything.
> > 
> 
> Ok, now I see. I think you're correct and I withdraw the patch.

I have Kirrill's

thp-reduce-indentation-level-in-change_huge_pmd.patch
thp-fix-madv_dontneed-vs-numa-balancing-race.patch
mm-drop-unused-pmdp_huge_get_and_clear_notify.patch
thp-fix-madv_dontneed-vs-madv_free-race.patch
thp-fix-madv_dontneed-vs-madv_free-race-fix.patch
thp-fix-madv_dontneed-vs-clear-soft-dirty-race.patch

scheduled for 4.12-rc1.  It sounds like
thp-fix-madv_dontneed-vs-madv_free-race.patch and
thp-fix-madv_dontneed-vs-madv_free-race.patch need to be boosted to
4.11 and stable?

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