Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping

[Yang Shi]

On 6/29/18 8:15 PM, Andrew Morton wrote:
> On Fri, 29 Jun 2018 19:28:15 -0700 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
>
> > > we're adding a bunch of code to 32-bit kernels which will never be
> > > executed.
> > >
> > > I'm thinking it would be better to be much more explicit with "#ifdef
> > > CONFIG_64BIT" in this code, rather than relying upon the above magic.
> > >
> > > But I tend to think that the fact that we haven't solved anything on
> > > locked vmas or on uprobed mappings is a shostopper for the whole
> > > approach :(
> > I agree it is not that perfect. But, it still could improve the most use
> > cases.
> Well, those unaddressed usecases will need to be fixed at some point.

Yes, definitely.

> What's our plan for that?

As I mentioned in the earlier email, locked and hugetlb cases might be 
able to be solved by separating vm_flags update and actual unmap. I will 
look into it further later.

From my point of view, uprobe mapping sounds not that vital.

> Would one of your earlier designs have addressed all usecases?  I
> expect the dumb unmap-a-little-bit-at-a-time approach would have?

Yes. The v1 design does unmap with holding write map_sem. So, the 
vm_flags update is not a problem.

Thanks,
Yang

> > For the locked vmas and hugetlb vmas, unmapping operations need modify
> > vm_flags. But, I'm wondering we might be able to separate unmap and
> > vm_flags update. Because we know they will be unmapped right away, the
> > vm_flags might be able to be updated in write mmap_sem critical section
> > before the actual unmap is called or after it. This is just off the top
> > of my head.
> >
> > For uprobed mappings, I'm not sure how vital it is to this case.
> >
> > Thanks,
> > Yang