Re: [PATCH v4] mm: per-thread vma caching

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 03 Mar 2014 19:13:30 -0800 Davidlohr Bueso <davidlohr@hp.com> wrote:

> On Mon, 2014-03-03 at 19:12 -0800, Andrew Morton wrote:
> > On Mon, 03 Mar 2014 18:42:33 -0800 Davidlohr Bueso <davidlohr@hp.com> wrote:
> > 
> > > On Mon, 2014-03-03 at 17:23 -0800, Andrew Morton wrote:
> > > > On Mon, 03 Mar 2014 16:59:38 -0800 Davidlohr Bueso <davidlohr@hp.com> wrote:
> > > > 
> > > > > > >...
> > > > > > >
> > > > > > > +static bool vmacache_valid(struct mm_struct *mm)
> > > > > > > +{
> > > > > > > +	struct task_struct *curr = current;
> > > > > > > +
> > > > > > > +	if (mm != curr->mm)
> > > > > > > +		return false;
> > > > > > 
> > > > > > What's going on here?  Handling a task poking around in someone else's
> > > > > > mm?  I'm thinking "__access_remote_vm", but I don't know what you were
> > > > > > thinking ;) An explanatory comment would be revealing.
> > > > > 
> > > > > I don't understand the doubt here. Seems like a pretty obvious thing to
> > > > > check -- yes it's probably unlikely but we certainly don't want to be
> > > > > validating the cache on an mm that's not ours... or are you saying it's
> > > > > redundant??
> > > > 
> > > > Well it has to be here for a reason and I'm wondering that that reason
> > > > is.  If nobody comes here with a foreign mm then let's remove it.
> > > 
> > > find_vma() can be called by concurrent threads sharing the mm->mmap_sem
> > > for reading, thus this check needs to be there.
> > 
> > Confused.  If the threads share mm->mmap_sem then they share mm and the
> > test will always be false?
> 
> Yes, I shortly realized that was silly... but I can say for sure it can
> happen and a quick qemu run confirms it. So I see your point as to
> asking why we need it, so now I'm looking for an explanation in the
> code.

Great, please do.  We may well find that we have buggy (or at least
inefficient) callers, which we can fix.



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