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

Suren Baghdasaryan
2024-12-19T16:14:40Z

On Thu, Dec 19, 2024 at 1:13 AM Peter Zijlstra <peterz@infradead.org> 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.
>
> > __mmap_new_vma
> > vma_set_range // installs new vma in the range
> > __mmap_complete
> > vms_complete_munmap_vmas // vmas are write-locked and detached
> > but it's too late
>
> But at this point that old vma really is unhooked, and the
> vma_write_start() here will ensure readers are gone and it will clear
> PTEs *again*.

So, to summarize, you want vma_start_write() and vma_mark_detached()
to be done when we are removing the vma from the tree, right?
Something like:

vma_start_write()
vma_iter_store()
vma_mark_detached()

And the race I described is not a real problem since the vma is still
in the tree, so gup_fast() does exactly that and will simply reinstall
the ptes.

>
>