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

To: Ingo Molnar <mingo@xxxxxxxxxx>
Subject: Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur
From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Sun, 8 Mar 2015 11:46:10 -0700
Cc: Mel Gorman <mgorman@xxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Aneesh Kumar <aneesh.kumar@xxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Linux-MM <linux-mm@xxxxxxxxx>, xfs@xxxxxxxxxxx, ppc-dev <linuxppc-dev@xxxxxxxxxxxxxxxx>
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On Sun, Mar 8, 2015 at 11:35 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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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