linux-kernel - Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

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Date:	Sun, 8 Mar 2015 11:46:10 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Mel Gorman <mgorman@...e.de>, Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, xfs@....sgi.com,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

On Sun, Mar 8, 2015 at 11:35 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>> As a second hack (not to be applied), could we change:
>>
>>  #define _PAGE_BIT_PROTNONE      _PAGE_BIT_GLOBAL
>>
>> to:
>>
>>  #define _PAGE_BIT_PROTNONE      (_PAGE_BIT_GLOBAL+1)
>>
>> to double check that the position of the bit does not matter?
>
> Agreed. We should definitely try that.

There's a second reason to do that, actually: the __supported_pte_mask
thing, _and_ the pageattr stuff in __split_large_page() etc play games
with _PAGE_GLOBAL. As does drivers/lguest for some reason.

So looking at this all, there's a lot of room for confusion with _PAGE_GLOBAL.

That kind of confusion would certainly explain the whole "the changes
_look_ like they do the same thing, but don't" - because of silly
semantic conflicts with PROTNONE vs GLOBAL.

                                Linus
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