linux-kernel - Re: [RFC 4/4] speculative pag fault

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Message-Id: <20091218153330.7f26a1bc.minchan.kim@barrios-desktop>
Date:	Fri, 18 Dec 2009 15:33:30 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, cl@...ux-foundation.org,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [RFC 4/4] speculative pag fault

On Fri, 18 Dec 2009 15:06:48 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> On Fri, 18 Dec 2009 14:54:49 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
> 
> > Hi, Kame. 
> > 
> Hi,
> 
> > On Fri, 18 Dec 2009 09:46:02 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > >  	if (flags & FAULT_FLAG_WRITE) {
> > >  		if (!pte_write(entry))
> > > 
> > 
> > I looked over the patch series and come up to one scenario.
> > 
> > CPU A				CPU 2
> > 
> > "Thread A reads page"
> > 		
> > do_page_fault
> > lookup_vma_cache
> > vma->cache_access++
> > 				"Thread B unmap the vma"
> > 
> > 				mm_write_lock
> > 				down_write(mm->mmap_sem)
> > 				mm->version++
> > 				do_munmap
> > 				wait_vmas_cache_access
> > 				wait_event_interruptible
> > mm_version_check fail
> > vma_release
> > wake_up(vma->cache_wait)
> > 				unmap_region
> > 				mm_write_unlock
> > mm_read_trylock
> > find_vma
> > !vma
> > bad_area
> > 				
> > As above scenario, Apparently, Thread A reads proper page in the vma at that time.
> > but it would meet the segment fault by speculative page fault. 
> > 
> Yes, It's intentional.
> 
> > Sorry that i don't have time to review more detail. 
> ya, take it easy. I'm not in hurry.
> 
> > If I miss something, Pz correct me. 
> > 
> 
> In multi-threaded application, mutual-exclusion of  memory-access v.s. munmap
> is the application's job. In above case, the application shouldn't unmap memory
> while it's access memory. (The application can be preempted at any point.)
> So, the kernel only have to take care of sanity of memory map status.
> In this case, no error in kernel's object. This is correct.

Ahhh. It's my fault. I need sleeping. :)
After take a enough rest, I will review continuosly. 

Thanks. Kame. 

> Thank you for your interests.
> 
> Regards,
> -Kame
> 
> 
> 
> 
> 


-- 
Kind regards,
Minchan Kim
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