linux-kernel - Re: [PATCH 4/5] mm: use paravirt friendly ops for NUMA hinting ptes

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Date:	Tue, 8 Apr 2014 18:21:28 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Linux-X86 <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Steven Noonan <steven@...inklabs.net>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] mm: use paravirt friendly ops for NUMA hinting ptes

On 08/04/14 14:09, Mel Gorman wrote:
> David Vrabel identified a regression when using automatic NUMA balancing
> under Xen whereby page table entries were getting corrupted due to the
> use of native PTE operations. Quoting him
> 
> 	Xen PV guest page tables require that their entries use machine
> 	addresses if the preset bit (_PAGE_PRESENT) is set, and (for
> 	successful migration) non-present PTEs must use pseudo-physical
> 	addresses.  This is because on migration MFNs in present PTEs are
> 	translated to PFNs (canonicalised) so they may be translated back
> 	to the new MFN in the destination domain (uncanonicalised).
> 
> 	pte_mknonnuma(), pmd_mknonnuma(), pte_mknuma() and pmd_mknuma()
> 	set and clear the _PAGE_PRESENT bit using pte_set_flags(),
> 	pte_clear_flags(), etc.
> 
> 	In a Xen PV guest, these functions must translate MFNs to PFNs
> 	when clearing _PAGE_PRESENT and translate PFNs to MFNs when setting
> 	_PAGE_PRESENT.
> 
> His suggested fix converted p[te|md]_[set|clear]_flags to using
> paravirt-friendly ops but this is overkill. He suggested an alternative of
> using p[te|md]_modify in the NUMA page table operations but this is does
> more work than necessary and would require looking up a VMA for protections.
> 
> This patch modifies the NUMA page table operations to use paravirt friendly
> operations to set/clear the flags of interest. Unfortunately this will take
> a performance hit when updating the PTEs on CONFIG_PARAVIRT but I do not
> see a way around it that does not break Xen.

Acked-by: David Vrabel <david.vrabel@...rix.com>

It passed my mprotect() PROT_NONE -> PROT_READ test case so

Tested-by: David Vrabel <david.vrabel@...rix.com>

I'll leave it up to the x86 maintainers to decide which fix to take.
This one or the more generic "x86: use pv-ops in
{pte,pmd}_{set,clear}_flags()"

David
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