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

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Date:	Thu, 7 Jan 2010 10:15:31 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.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, 7 Jan 2010, Peter Zijlstra wrote:
> 
> Well, with that sync_vma() thing I posted the other day all the
> speculative page fault needs is a write to current->fault_vma, the ptl
> and an O(nr_threads) loop on unmap() for file vmas -- aside from writing
> the pte itself of course.

How do you handle the race of

	fault handler:			munmap

	look up vma without locks

	.. interrupt happens or
	  something delays things ..
					remove the vma from the list
					sync_vma()

	current->fault_vma = vma

etc?

Maybe I'm missing something, but quite frankly, the above looks pretty 
damn fundamental. Something needs to take a lock. If you get rid of the 
mmap_sem, you need to replace it with another lock.

There's no sane way to "look up vma and atomically mark it as busy" 
without locks. You can do it with extra work, ie something like

	look up vma without locks
	current->fault_vma = vma
	smp_mb();
	check that the vma is still on the list

(with he appropriate barriers on the munmap side too, of course) where you 
avoid the lock by basically turning it into an ordering problem, but it 
ends up being pretty much as expensive anyway for single threads.

And for lots and lots of threads, you now made that munmap be pretty 
expensive.

		Linus
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