linux-kernel - Re: [PATCH] mm: Check if PTE is already allocated during page fault

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Message-ID: <BANLkTi=+fGe-hrV3c8r2jKzWG2BHU0GsFA@mail.gmail.com>
Date:	Fri, 22 Apr 2011 09:54:24 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Mel Gorman <mgorman@...e.de>, akpm@...ux-foundation.org,
	raz ben yehuda <raziebe@...il.com>, riel@...hat.com,
	kosaki.motohiro@...fujitsu.com,
	lkml <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	stable@...nel.org
Subject: Re: [PATCH] mm: Check if PTE is already allocated during page fault

Hi Andrea,

On Fri, Apr 22, 2011 at 1:14 AM, Andrea Arcangeli <aarcange@...hat.com> wrote:
> On Thu, Apr 21, 2011 at 05:00:57PM +0100, Mel Gorman wrote:
>> If you want to create a new patch with either your comment or mine
>> (whichever you prefer) I'll add my ack. I'm about to drop offline
>> for a few days but if it's still there Tuesday, I'll put together an
>> appropriate patch and submit. I'd keep it separate from the other patch
>> because it's a performance fix (which I'd like to see in -stable) where
>> as this is more of a cleanup IMO.
>
> I think the older patch should have more priority agreed. This one may
> actually waste cpu cycles overall, rather than saving them, it
> shouldn't be a common occurrence.
>
> From a code consistency point of view maybe we should just implement a
> pte_alloc macro (to put after pte_alloc_map) and use it in both
> places, and hide the glory details of the unlikely in the macro. When
> implementing pte_alloc, I suggest also adding unlikely to both, I mean
> we added unlikely to the fast path ok, but __pte_alloc is orders of
> magnitude less likely to fail than pte_none, and it still runs 1 every
> 512 4k page faults, so I think __pte_alloc deserves an unlikely too.
>
> Minchan, you suggested this cleanup, so I suggest you to send a patch,
> but if you're busy we can help.

It's no problem to send a patch but I can do it at out-of-office time.
Maybe weekend. :)
Before doing that, let's clear the point. You mentioned  it shouldn't
be a common occurrence but you are suggesting we should do for code
consistency POV. Am I right?

>
> Thanks!
> Andrea
>



-- 
Kind regards,
Minchan Kim
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