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

Suren Baghdasaryan
2024-12-18T19:07:49Z

On Wed, Dec 18, 2024 at 11:00 AM 'Liam R. Howlett' via kernel-team
<kernel-team@android.com> wrote:
>
> * Suren Baghdasaryan <surenb@google.com> [241218 12:58]:
> > On Wed, Dec 18, 2024 at 9:44 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Wed, Dec 18, 2024 at 09:36:42AM -0800, Suren Baghdasaryan wrote:
> > >
> > > > > You will not. vms_complete_munmap_vmas() will call remove_vma() to
> > > > > remove PTEs IIRC, and if you do start_write() and detach() before
> > > > > dropping mmap_lock_write, you should be good.
> > > >
> > > > Ok, I think we will have to move mmap_write_downgrade() inside
> > > > vms_complete_munmap_vmas() to be called after remove_vma().
> > > > vms_clear_ptes() is using vmas, so we can't move remove_vma() before
> > > > mmap_write_downgrade().
> > >
> > > Why ?!
> > >
> > > vms_clear_ptes() and remove_vma() are fine where they are -- there is no
> > > concurrency left at this point.
> > >
> > > Note that by doing vma_start_write() inside vms_complete_munmap_vmas(),
> > > which is *after* the vmas have been unhooked from the mm, you wait for
> > > any concurrent user to go away.
> > >
> > > And since they're unhooked, there can't be any new users.
> > >
> > > So you're the one and only user left, and code is fine the way it is.
> >
> > Ok, let me make sure I understand this part of your proposal. From
> > your earlier email:
> >
> > @@ -1173,6 +1173,11 @@ static void vms_complete_munmap_vmas(struct
> > vma_munmap_struct *vms,
> > struct vm_area_struct *vma;
> > struct mm_struct *mm;
> >
> > + mas_for_each(mas_detach, vma, ULONG_MAX) {
> > + vma_start_write(next);
> > + vma_mark_detached(next, true);
> > + }
> > +
> > mm = current->mm;
> > mm->map_count -= vms->vma_count;
> > mm->locked_vm -= vms->locked_vm;
> >
> > This would mean:
> >
> > vms_complete_munmap_vmas
> > vma_start_write
> > vma_mark_detached
> > mmap_write_downgrade
> > vms_clear_ptes
> > remove_vma
> >
> > And remove_vma will be just freeing the vmas. Is that correct?
> > I'm a bit confused because the original thinking was that
> > vma_mark_detached() would drop the last refcnt and if it's 0 we would
> > free the vma right there. If that's still what we want to do then I
> > think the above sequence should look like this:
> >
> > vms_complete_munmap_vmas
> > vms_clear_ptes
> > remove_vma
> > vma_start_write
> > vma_mark_detached
> > mmap_write_downgrade
> >
> > because vma_start_write+vma_mark_detached should be done under mmap_write_lock.
> > Please let me know which way you want to move forward.
> >
>
> Are we sure we're not causing issues with the MAP_FIXED path here?
>
> With the above change, we'd be freeing the PTEs before marking the vmas
> as detached or vma_start_write().

IIUC when we call vms_complete_munmap_vmas() all vmas inside
mas_detach have been already write-locked, no?

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