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

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Message-ID: <20141024155101.GE21513@worktop.programming.kicks-ass.net>
Date:	Fri, 24 Oct 2014 17:51:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
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, Oct 24, 2014 at 10:16:24AM -0500, Christoph Lameter wrote:

> Hmmm... One optimization to do before we get into these changes is to work
> on allowing the dropping of mmap_sem before we get to sleeping and I/O and
> then reevaluate when I/O etc is complete? This is probably the longest
> hold on mmap_sem that is also frequent. Then it may be easier to use
> standard RCU later.

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.

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.

Also, there is a lot of possible blocking, there's lock_page,
page_mkwrite() -- which ends up calling into the dirty throttle etc. We
could not possibly retry on all that, the error paths involved would be
horrible for one.

That said, there's a fair bit of code that does allow the retry, and I
think most fault paths actually do the retry on IO.
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