Re: [RFC 4/4] speculative pag fault

Minchan Kim
2009-12-18T06:40:21Z

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

> On Fri, 18 Dec 2009 14:54:49 +0900
> Minchan Kim <minchan.kim@gmail.com> wrote:
>
> > Hi, Kame.
> >
> Hi,
>
> > On Fri, 18 Dec 2009 09:46:02 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.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