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Subject: Re: [PATCH 08/21] mm: Preemptible mmu_gather
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>, Avi Kivity <avi@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, Rik van Riel <riel@redhat.com>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
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        linux-mm@kvack.org, Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        David Miller <davem@davemloft.net>,
        Hugh Dickins <hugh.dickins@tiscali.co.uk>, Mel Gorman <mel@csn.ul.ie>,
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        Yanmin Zhang <yanmin_zhang@linux.intel.com>,
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        Jeff Dike <jdike@addtoit.com>, Tony Luck <tony.luck@intel.com>
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Date: Mon, 29 Nov 2010 21:47:02 +0100
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On Mon, 2010-11-29 at 11:53 +0900, KAMEZAWA Hiroyuki wrote:
> On Fri, 26 Nov 2010 15:38:51 +0100
> Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 
> > Make mmu_gather preemptible by using a small on stack list and use
> > an option allocation to speed things up.
> > 
> > Preemptible mmu_gather is desired in general and usable once
> > i_mmap_lock becomes a mutex. Doing it before the mutex conversion
> > saves us from having to rework the code by moving the mmu_gather
> > bits inside the i_mmap_lock.
> > 
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: David Miller <davem@davemloft.net>
> > Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > Cc: Russell King <rmk@arm.linux.org.uk>
> > Cc: Paul Mundt <lethal@linux-sh.org>
> > Cc: Jeff Dike <jdike@addtoit.com>
> > Cc: Tony Luck <tony.luck@intel.com>
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> 
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> Interesting, Hmm, how about using the 1st freed pages as tlb->pages
> rathet than calling alloc_page() ? no benefits ?

We could try that, but we need to be careful there, you need to wait
till after the TLB invalidate and possibly an RCU period for the
page-directory pages (sparc/powerpc).

So doing an optimistic allocation before the TLB invalidate seems saner.

Also, see patch 20.