linux-kernel - Re: [PATCH] mm: per-thread vma caching

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Message-ID: <CA+55aFw1_Ecbjjv9vijj3o46mkq3NrJn0X-FnbpCGBZG2=NuOA@mail.gmail.com>
Date:	Fri, 21 Feb 2014 10:13:54 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: per-thread vma caching

On Thu, Feb 20, 2014 at 9:28 PM, Davidlohr Bueso <davidlohr@...com> wrote:
> From: Davidlohr Bueso <davidlohr@...com>
>
> This patch is a continuation of efforts trying to optimize find_vma(),
> avoiding potentially expensive rbtree walks to locate a vma upon faults.

Ok, so I like this one much better than the previous version.

However, I do wonder if the per-mm vmacache is actually worth it.
Couldn't the per-thread one replace it entirely?

Also, the hash you use for the vmacache index is *particularly* odd.

        int idx =  (addr >> 10) & 3;

you're using the top two bits of the address *within* the page.
There's a lot of places that round addresses down to pages, and in
general it just looks really odd to use an offset within a page as an
index, since in some patterns (linear accesses, whatever), the page
faults will always be to the beginning of the page, so index 0 ends up
being special.

What am I missing?

               Linus
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