Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

[Peter Zijlstra]

On Thu, Dec 19, 2024 at 10:13:34AM +0100, Peter Zijlstra wrote:
> On Wed, Dec 18, 2024 at 01:53:17PM -0800, Suren Baghdasaryan wrote:
> 
> > Ah, ok I see now. I completely misunderstood what for_each_vma_range()
> > was doing.
> > 
> > Then I think vma_start_write() should remain inside
> > vms_gather_munmap_vmas() and all vmas in mas_detach should be
> 
> No, it must not. You really are not modifying anything yet (except the
> split, which we've already noted mark write themselves).
> 
> > write-locked, even the ones we are not modifying. Otherwise what would
> > prevent the race I mentioned before?
> > 
> > __mmap_region
> >     __mmap_prepare
> >         vms_gather_munmap_vmas // adds vmas to be unmapped into mas_detach,
> >                                                       // some locked
> > by __split_vma(), some not locked
> > 
> >                                      lock_vma_under_rcu()
> >                                          vma = mas_walk // finds
> > unlocked vma also in mas_detach
> >                                          vma_start_read(vma) //
> > succeeds since vma is not locked
> >                                          // vma->detached, vm_start,
> > vm_end checks pass
> >                                      // vma is successfully read-locked
> > 
> >        vms_clean_up_area(mas_detach)
> >             vms_clear_ptes
> >                                      // steps on a cleared PTE
> 
> So here we have the added complexity that the vma is not unhooked at
> all. Is there anything that would prevent a concurrent gup_fast() from
> doing the same -- touch a cleared PTE?
> 
> AFAICT two threads, one doing overlapping mmap() and the other doing
> gup_fast() can result in exactly this scenario.
> 
> If we don't care about the GUP case, when I'm thinking we should not
> care about the lockless RCU case either.

Also, at this point we'll just fail to find a page, and that is nothing
special. The problem with accessing an unmapped VMA is that the
page-table walk will instantiate page-tables.

Given this is an overlapping mmap -- we're going to need to those
page-tables anyway, so no harm done.

Only after the VMA is unlinked must we ensure we don't accidentally
re-instantiate page-tables.