linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Date:	Thu, 07 Jan 2010 22:44:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

On Thu, 2010-01-07 at 10:44 -0800, Linus Torvalds wrote:
> 
> On Thu, 7 Jan 2010, Linus Torvalds wrote:
> > 
> > For example: there's no real reason why we take mmap_sem for writing when 
> > extending an existing vma. And while 'brk()' is a very oldfashioned way of 
> > doing memory management, it's still quite common. So rather than looking 
> > at subtle lockless algorithms, why not look at doing the common cases of 
> > an extending brk? Make that one take the mmap_sem for _reading_, and then 
> > do the extending of the brk area with a simple cmpxchg or something?
> 
> I didn't use cmpxchg, because we actually want to update both 
> 'current->brk' _and_ the vma->vm_end atomically, so here's a totally 
> untested patch that uses the page_table_lock spinlock for it instead (it 
> could be a new spinlock, not worth it).
> 
> It's also totally untested and might be horribly broken. But you get the 
> idea.
> 
> We could probably do things like this in regular mmap() too for the 
> "extend a mmap" case. brk() is just especially simple.

I haven't yet looked at the patch, but isn't expand_stack() kinda like
what you want? That serializes using anon_vma_lock().

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