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

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Message-ID: <20100107192035.GO6764@linux.vnet.ibm.com>
Date:	Thu, 7 Jan 2010 11:20:35 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	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, Jan 07, 2010 at 10:44:13AM -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.

One question on the final test...

> 		Linus
> 
> ---
>  mm/mmap.c |   89 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-------
>  1 files changed, 79 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index ee22989..3d07e5f 100644

[ . . . ]

> +	if (!vma)
> +		goto slow_case;
> +
> +	spin_lock(&mm->page_table_lock);
> +	if (vma->vm_end == cur_brk) {
> +		vma->vm_end = brk;
> +		mm->brk = brk;
> +		cur_brk = brk;
> +	}
> +	spin_unlock(&mm->page_table_lock);
> +
> +	if (cur_brk != brk)

Can this be "if (cur_brk < brk)"?  Seems like it should, given the
earlier tests, but I don't claim to understand the VM code.

							Thanx, Paul

> +		goto slow_case;
> +
> +out:
> +	up_read(&mm->mmap_sem);
> +	return cur_brk;
> +
> +slow_case:
> +	up_read(&mm->mmap_sem);
> +	return slow_brk(brk);
> +}
> +
>  #ifdef DEBUG_MM_RB
>  static int browse_rb(struct rb_root *root)
>  {
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