linux-kernel - Re: [PATCH 4/4] mm: numa: Do not mark PTEs pte

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Message-ID: <20141002151807.GA13668@node.dhcp.inet.fi>
Date:	Thu, 2 Oct 2014 18:18:07 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Michel Lespinasse <walken@...gle.com>,
	Kirill A Shutemov <kirill.shutemov@...ux.intel.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] mm: numa: Do not mark PTEs pte_numa when splitting
 huge pages

On Thu, Oct 02, 2014 at 02:29:18PM +0100, Mel Gorman wrote:
> This patch reverts 1ba6e0b50b ("mm: numa: split_huge_page: transfer the
> NUMA type from the pmd to the pte"). If a huge page is being split due
> a protection change and the tail will be in a PROT_NONE vma then NUMA
> hinting PTEs are temporarily created in the protected VMA.
> 
>  VM_RW|VM_PROTNONE
> |-----------------|
>       ^
>       split here
> 
> In the specific case above, it should get fixed up by change_pte_range()
> but there is a window of opportunity for weirdness to happen. Similarly,
> if a huge page is shrunk and split during a protection update but before
> pmd_numa is cleared then a pte_numa can be left behind.
> 
> Instead of adding complexity trying to deal with the case, this patch
> will not mark PTEs NUMA when splitting a huge page. NUMA hinting faults
> will not be triggered which is marginal in comparison to the complexity
> in dealing with the corner cases during THP split.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>

I think it deserves a comment to avoid re-introducing NUMA type
transferring.

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov
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