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

[Davidlohr Bueso]

On Mon, 2014-03-03 at 17:23 -0800, Andrew Morton wrote:
> On Mon, 03 Mar 2014 16:59:38 -0800 Davidlohr Bueso <davidlohr@xxxxxx> 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.

Thanks,
Davidlohr

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