linux-kernel - Re: [PATCH 14/15] mm: fix TLB flush race between migration, and change_protection

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Date:	Fri, 06 Dec 2013 16:21:21 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	Mel Gorman <mgorman@...e.de>, Alex Thorlton <athorlton@....com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/15] mm: fix TLB flush race between migration, and change_protection_range

On 12/06/2013 03:32 PM, Christoph Lameter wrote:
> On Fri, 6 Dec 2013, Rik van Riel wrote:
>>
>> The basic race looks like this:
>>
>> CPU A			CPU B			CPU C
>>
>> 						load TLB entry
>> make entry PTE/PMD_NUMA
>> 			fault on entry
>> 						read/write old page
>> 			start migrating page
> 
> When you start migrating a page a special page migration entry is
> created that will trap all accesses to the page. You can safely flush when
> the migration entry is there. Only allow a new PTE/PMD to be put there
> *after* the tlb flush.

A PROT_NONE or NUMA pte is just as effective as a migration pte.
The only problem is, the TLB flush was not always done...

> 
>> 			change PTE/PMD to new page
> 
> Dont do that. We have migration entries for a reason.

We do not have migration entries for hugepages, do we?

>> 						read/write old page [*]
> 
> Should cause a page fault which should put the process to sleep. Process
> will safely read the page after the migration entry is removed.
> 
>> flush TLB
> 
> Establish the new PTE/PMD after the flush removing the migration pte
> entry and thereby avoiding the race.

That is what this patch does.

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