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

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Message-ID: <00000142d84d07dc-4a095b73-a35b-4c74-8cc0-a57283ef308f-000000@email.amazonses.com>
Date:	Mon, 9 Dec 2013 16:59:56 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Mel Gorman <mgorman@...e.de>
cc:	Rik van Riel <riel@...hat.com>, 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 Mon, 9 Dec 2013, Mel Gorman wrote:

> I looked at what would be required to implement migration entry support for
> PMDs. It's major surgery because we do not have something like swap-like
> entries to use at that page table level. It looked like it would require
> inserting a fake entry (easiest would be to point to a global page) that
> all page table walkers would recognise, blocking on it and teaching every
> page table walker to get it right.

Well something needs to cause a fault and stop accesses to the page.

> One can't do something simple like clear the entry out because then the
> no page handlers for GUP or faults insert the zero page behind and it goes
> to hell and we can't hold the page table lock across the migration copy.

Right you need to have special migration entry there. Same as for regular
sized pages.


> The patch flushes the TLBs as it is and future accesses are help up in the
> NUMA hinting fault handler. It's functionally similar to having a migration
> entry albeit it is special cased to handle just automatic NUMA balancing

Hmmm... Hopefully that will work. I'd rather see a clean extension of what
we use for regular pages. If we add functionality to huge pages to operate
more like regular ones then this could be an issue.

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