Re: [PATCH] mm: protect against concurrent vma expansion

[Simon Jeons]

On Thu, 2013-01-03 at 16:50 -0800, Michel Lespinasse wrote:
> On Thu, Jan 3, 2013 at 4:40 PM, Simon Jeons <simon.jeons@xxxxxxxxx> wrote:
> > On Wed, 2012-12-19 at 19:01 -0800, Michel Lespinasse wrote:
> >> Hi Simon,
> >>
> >> On Wed, Dec 19, 2012 at 5:56 PM, Simon Jeons <simon.jeons@xxxxxxxxx> wrote:
> >> > One question.
> >> >
> >> > I found that mainly callsite of expand_stack() is #PF, but it holds
> >> > mmap_sem each time before call expand_stack(), how can hold a *shared*
> >> > mmap_sem happen?
> >>
> >> the #PF handler calls down_read(&mm->mmap_sem) before calling expand_stack.
> >>
> >> I think I'm just confusing you with my terminology; shared lock ==
> >> read lock == several readers might hold it at once (I'd say they share
> >> it)
> >
> > Sorry for my late response.
> >
> > Since expand_stack() will modify vma, then why hold a read lock here?
> 
> Well, it'd be much nicer if we had a write lock, I think. But, we
> didn't know when taking the lock that we'd end up having to expand
> stacks.
> 
> What happens is that page faults don't generally modify vmas, so they
> get a read lock (just to know what vma the fault is happening in) and
> then fault in the page.
> 

Thanks for your quick explanation. 

> expand_stack() is the one exception to that - after getting the read
> lock as usual, we notice that the fault is not in any vma right now,
> but it's close enough to an expandable vma.

If this senario only occur for userspace stack?

> 


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