Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure

[Peter Zijlstra]

On Tue, Aug 29, 2017 at 03:18:25PM +0200, Laurent Dufour wrote:
> On 29/08/2017 14:04, Peter Zijlstra wrote:
> > On Tue, Aug 29, 2017 at 09:59:30AM +0200, Laurent Dufour wrote:
> >> On 27/08/2017 02:18, Kirill A. Shutemov wrote:
> >>>> +
> >>>> +	if (unlikely(!vma->anon_vma))
> >>>> +		goto unlock;
> >>>
> >>> It deserves a comment.
> >>
> >> You're right I'll add it in the next version.
> >> For the record, the root cause is that __anon_vma_prepare() requires the
> >> mmap_sem to be held because vm_next and vm_prev must be safe.
> > 
> > But should that test not be:
> > 
> > 	if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
> > 		goto unlock;
> > 
> > Because !anon vmas will never have ->anon_vma set and you don't want to
> > exclude those.
> 
> Yes in the case we later allow non anonymous vmas to be handled.
> Currently only anonymous vmas are supported so the check is good enough,
> isn't it ?

That wasn't at all clear from reading the code. This makes it clear
->anon_vma is only ever looked at for anonymous.

And like Kirill says, we _really_ should start allowing some (if not
all) vm_ops. Large file based mappings aren't particularly rare.

I'm not sure we want to introduce a white-list or just bite the bullet
and audit all ->fault() implementations. But either works and isn't
terribly difficult, auditing all is more work though.