Re: [PATCH 3/3] mm/mmap: Regression fix for unmapped_area{_topdown}

[Edgecombe, Rick P]

On Fri, 2023-04-14 at 14:07 -0400, Liam R. Howlett wrote:
> * Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> [230414 13:53]:
> > On Fri, 2023-04-14 at 13:29 -0400, Liam R. Howlett wrote:
> > > * Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> [230414 13:26]:
> > > > * Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> [230414
> > > > 12:27]:
> > > > > On Fri, 2023-04-14 at 10:57 -0400, Liam R. Howlett wrote:<br>
> > > > > > +       tmp = mas_next(&mas, ULONG_MAX);
> > > > > > +       if (tmp && (tmp->vm_flags & VM_GROWSDOWN)) {
> > > > > 
> > > > > Why also check VM_GROWSDOWN here (and VM_GROWSUP below)?
> > > > > vm_start/end_gap() already have checks inside.
> > > > 
> > > > An artifact of a plan that was later abandoned.
> > > > 
> > > > > 
> > > > > > +               if (vm_start_gap(tmp) < gap + length - 1) {
> > > > > > +                       low_limit = tmp->vm_end;
> > > > > > +                       mas_reset(&mas);
> > > > > > +                       goto retry;
> > > > > > +               }
> > > > > > +       } else {
> > > > > > +               tmp = mas_prev(&mas, 0);
> > > > > > +               if (tmp && (tmp->vm_flags & VM_GROWSUP) &&
> > > > > > +                   vm_end_gap(tmp) > gap) {
> > > > > > +                       low_limit = vm_end_gap(tmp); 
> > > > > > +                       mas_reset(&mas);
> > > > > > +                       goto retry; 
> > > > > > +               }
> > > > > > +       } 
> > > > > > +
> > > > > 
> > > > > Could it be like this?
> > > > 
> > > > Yes, I'll make this change.  Thanks for the suggestion.
> > > 
> > > 
> > > Wait, I like how it is.
> > > 
> > > In my version, if there is a stack that is VM_GROWSDOWN there,
> > > but
> > > does
> > > not intercept the gap, then I won't check the prev.. in yours, we
> > > will
> > > never avoid checking prev.
> > 
> > Hmm, I see. I guess I'm thinking ahead a bit to adding the shadow
> > stack
> > guard gap, but I can always add to these vm_flags checks.
> > 
> > But are you sure this optimization is even possible? The old
> > vma_compute_gap() had this comment:
> > /*
> >   * Note: in the rare case of a VM_GROWSDOWN above a VM_GROWSUP, we
> >   * allow two stack_guard_gaps between them here, and when choosing
> >   * an unmapped area; whereas when expanding we only require one.
> >   * That's a little inconsistent, but keeps the code here simpler.
> >   */
> 
> I didn't think this was possible.  ia64 (orphaned in 96ec72a3425d)
> did
> this.

Ah, ok.

> 
> > 
> > Assuming this is a real scenario, if you have VM_GROWSDOWN above
> > and
> > VM_GROWSUP below, don't you need to check the gaps for above and
> > below?
> > Again thinking about adding shadow stack guard pages, something
> > like
> > that could be a more common scenario. Not that you need to fix my
> > out
> > of tree issues, but I would probably need to adjust it to check
> > both
> > directions.
> > 
> > I guess there is no way to embed this inside maple tree search so
> > we
> > don't need to retry? (sorry if this is a dumb question, it's an
> > opaque
> > box to me).
> 
> Absolutely, and I'm working on this as well, but right now I'm trying
> to fix my regression for this and past releases.  Handling this in
> the
> maple tree is more involved and so there's more risk.

Ok, thanks. It looks good to me in that respect.