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

Suren Baghdasaryan
2024-12-18T21:53:33Z

On Wed, Dec 18, 2024 at 12:38 PM Liam R. Howlett
<Liam.Howlett@oracle.com> wrote:
>
> * Suren Baghdasaryan <surenb@google.com> [241218 15:01]:
> > On Wed, Dec 18, 2024 at 11:38 AM 'Liam R. Howlett' via kernel-team
> > <kernel-team@android.com> wrote:
> > >
> > > * Suren Baghdasaryan <surenb@google.com> [241218 14:29]:
> > > > On Wed, Dec 18, 2024 at 11:07 AM Suren Baghdasaryan <surenb@google.com> wrote:
> > > > >
> > > > > 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?
> > >
> > > That's the way it is today - but I thought you were moving the lock to
> > > the complete stage, not adding a new one? (why add a new one otherwise?)
> >
> > Is my understanding correct that mas_detach is populated by
> > vms_gather_munmap_vmas() only with vmas that went through
> > __split_vma() (and were write-locked there)? I don't see any path that
> > would add any other vma into mas_detach but maybe I'm missing
> > something?
>
> No, that is not correct.
>
> vms_gather_munmap_vmas() calls split on the first vma, then adds all
> vmas that are within the range of the munmap() call. Potentially
> splitting the last vma and adding that in the
> "if (next->vm_end > vms->end)" block.
>
> Sometimes this is a single vma that gets split twice, sometimes no
> splits happen and entire vmas are unmapped, sometimes it's just one vma
> that isn't split.
>
> My observation is the common case is a single vma, but besides that we
> see 3, and sometimes 7 at a time, but it could be any number of vmas and
> not all of them are split.
>
> There is a loop for_each_vma_range() that does:
>
> vma_start_write(next);
> mas_set(mas_detach, vms->mas_count++);
> mas_store_gfp(mas_detach, next, GFP_KERNEL);

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
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
__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

>
>
> >
> > >
> > > >
> > > > Yeah, I think we can simply do this:
> > > >
> > > > vms_complete_munmap_vmas
> > > > vms_clear_ptes
> > > > remove_vma
> > > > vma_mark_detached
> > > > mmap_write_downgrade
> > > >
> > > > If my assumption is incorrect, assertion inside vma_mark_detached()
> > > > should trigger. I tried a quick test and so far nothing exploded.
> > > >
> > >
> > > If they are write locked, then the page faults are not a concern. There
> > > is also the rmap race that Jann found in mmap_region() [1]. This is
> > > probably also fine since you are keeping the write lock in place earlier
> > > on in the gather stage. Note the ptes will already be cleared by the
> > > time vms_complete_munmap_vmas() is called in this case.
> > >
> > > [1] https://lore.kernel.org/all/CAG48ez0ZpGzxi=-5O_uGQ0xKXOmbjeQ0LjZsRJ1Qtf2X5eOr1w@mail.gmail.com/
> > >
> > > Thanks,
> > > Liam
> > >
> > > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
> > >