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Date: Fri, 24 Oct 2014 12:08:50 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
Subject: Re: [RFC][PATCH 4/6] SRCU free VMAs
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References: <20141020215633.717315139@infradead.org> <20141020222841.419869904@infradead.org> <CA+55aFwd04q+O5ejbmDL-H7_GB6DEBMiiHkn+2R1u4uWxfDO9w@mail.gmail.com> <20141021080740.GJ23531@worktop.programming.kicks-ass.net> <alpine.DEB.2.11.1410241003430.29419@gentwo.org>
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Paul McKenney <paulmck@linux.vnet.ibm.com>, Thomas Gleixner <tglx@linutronix.de>, Andrew Morton <akpm@linux-foundation.org>, Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>, Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@redhat.com>, Minchan Kim <minchan@kernel.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Al Viro <viro@zeniv.linux.org.uk>, Lai Jiangshan <laijs@cn.fujitsu.com>, Davidlohr Bueso <dave@stgolabs.net>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>

On Fri, 24 Oct 2014, Peter Zijlstra wrote:

> The hold time isn't relevant, in fact breaking up the mmap_sem such that
> we require multiple acquisitions will just increase the cacheline
> bouncing.

Well this wont be happening anymore once you RCUify the stuff. If you go
to sleep then its best to release mmap_sem and then the bouncing wont
matter.

Dropping mmap_sem there will also expose you to races you will see later
too when you RCUify the code paths. That way those can be deal with
beforehand.

> Also I think it makes more sense to continue an entire fault operation,
> including blocking, if at all possible. Every retry will just waste more
> time.

Ok then dont retry. Just drop mmap_sem before going to sleep. When you
come back evaluate the situation and if we can proceed do so otherwise
retry.

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