linux-kernel - Re: [RFC][PATCH 4/6] SRCU free VMAs

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Message-ID: <alpine.DEB.2.11.1410241206001.30305@gentwo.org>
Date:	Fri, 24 Oct 2014 12:08:50 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Minchan Kim <minchan@...nel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC][PATCH 4/6] SRCU free VMAs

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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