linux-kernel - Re: [RFC PATCH] asynchronous page fault.

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Date:	Mon, 28 Dec 2009 12:13:18 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"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>,
	cl@...ux-foundation.org
Subject: Re: [RFC PATCH] asynchronous page fault.

On Mon, 28 Dec 2009 08:28:39 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-12-28 10:05:14]:

> >   - rb-tree's rb_left and rb_right don't points to memory other than
> >     rb-tree. (or NULL)  And vmas are not freed/reused while rcu_read_lock().
> >     Then, we don't dive into unknown memory.
> >   - Then, we can skip rcu_assign_pointer().
> >
> 
> We can, but the data being on read-side is going to be out-of-date
> more than without the use of rcu_assign_pointer(). Do we need variants
> like to rcu_rb_next() to avoid overheads for everyone?
> 
I myself can't know how often out-of-date data can be seen (because I use x86).

But, I feel that we don't see broken tree so often. Because...
  - Single-threaded apps never see broken tree.
  - Even if rb-tree modification frequently happens, tree rotation is not
    very often and sub-trees tend to be stable as a chunk.

Hmm, adding barrier like this ?

static inline void
__vma_unlink(struct mm_struct *mm, struct vm_area_struct *vma,
                struct vm_area_struct *prev)
{
        prev->vm_next = vma->vm_next;
        rb_erase(&vma->vm_rb, &mm->mm_rb);
        if (mm->mmap_cache == vma)
                mm->mmap_cache = prev;
	smp_wb(); <==============================================(new)
}



Regards,
-Kame

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