Re: [PATCH v6 7/8] fs/proc/task_mmu: read proc/pid/maps under per-vma lock

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jul 9, 2025 at 10:47 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Wed, Jul 9, 2025 at 4:12 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
> >
> > * Suren Baghdasaryan <surenb@google.com> [250709 11:06]:
> > > On Wed, Jul 9, 2025 at 3:03 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> > > >
> > > > On 7/9/25 16:43, Suren Baghdasaryan wrote:
> > > > > On Wed, Jul 9, 2025 at 1:57 AM Vlastimil Babka <vbabka@suse.cz> wrote:
> > > > >>
> > > > >> On 7/8/25 01:10, Suren Baghdasaryan wrote:
> > > > >> >>> +     rcu_read_unlock();
> > > > >> >>> +     vma = lock_vma_under_mmap_lock(mm, iter, address);
> > > > >> >>> +     rcu_read_lock();
> > > > >> >> OK I guess we hold the RCU lock the whole time as we traverse except when
> > > > >> >> we lock under mmap lock.
> > > > >> > Correct.
> > > > >>
> > > > >> I wonder if it's really necessary? Can't it be done just inside
> > > > >> lock_next_vma()? It would also avoid the unlock/lock dance quoted above.
> > > > >>
> > > > >> Even if we later manage to extend this approach to smaps and employ rcu
> > > > >> locking to traverse the page tables, I'd think it's best to separate and
> > > > >> fine-grain the rcu lock usage for vma iterator and page tables, if only to
> > > > >> avoid too long time under the lock.
> > > > >
> > > > > I thought we would need to be in the same rcu read section while
> > > > > traversing the maple tree using vma_next() but now looking at it,
> > > > > maybe we can indeed enter only while finding and locking the next
> > > > > vma...
> > > > > Liam, would that work? I see struct ma_state containing a node field.
> > > > > Can it be freed from under us if we find a vma, exit rcu read section
> > > > > then re-enter rcu and use the same iterator to find the next vma?
> > > >
> > > > If the rcu protection needs to be contigous, and patch 8 avoids the issue by
> > > > always doing vma_iter_init() after rcu_read_lock() (but does it really avoid
> > > > the issue or is it why we see the syzbot reports?) then I guess in the code
> > > > quoted above we also need a vma_iter_init() after the rcu_read_lock(),
> > > > because although the iterator was used briefly under mmap_lock protection,
> > > > that was then unlocked and there can be a race before the rcu_read_lock().
> > >
> > > Quite true. So, let's wait for Liam's confirmation and based on his
> > > answer I'll change the patch by either reducing the rcu read section
> > > or adding the missing vma_iter_init() after we switch to mmap_lock.
> >
> > You need to either be under rcu or mmap lock to ensure the node in the
> > maple state hasn't been freed (and potentially, reallocated).
> >
> > So in this case, in the higher level, we can hold the rcu read lock for
> > a series of walks and avoid re-walking the tree then the performance
> > would be better.
>
> Got it. Thanks for confirming!
>
> >
> > When we return to userspace, then we should drop the rcu read lock and
> > will need to vma_iter_set()/vma_iter_invalidate() on return.  I thought
> > this was being done (through vma_iter_init()), but syzbot seems to
> > indicate a path that was missed?
>
> We do that in m_start()/m_stop() by calling
> lock_vma_range()/unlock_vma_range() but I think I have two problems
> here:
> 1. As Vlastimil mentioned I do not reset the iterator when falling
> back to mmap_lock and exiting and then re-entering rcu read section;
> 2. I do not reset the iterator after exiting rcu read section in
> m_stop() and re-entering it in m_start(), so the later call to
> lock_next_vma() might be using an iterator with a node that was freed
> (and possibly reallocated).
>
> >
> > This is the same thing that needed to be done previously with the mmap
> > lock, but now under the rcu lock.
> >
> > I'm not sure how to mitigate the issue with the page table, maybe we
> > guess on the number of vmas that we were doing for 4k blocks of output
> > and just drop/reacquire then.  Probably a problem for another day
> > anyways.
> >
> > Also, I think you can also change the vma_iter_init() to vma_iter_set(),
> > which is slightly less code under the hood.  Vlastimil asked about this
> > and it's probably a better choice.
>
> Ack.
> I'll update my series with these fixes and all comments I received so
> far, will run the reproducers to confirm no issues and repost them
> later today.

I have the patchset ready but would like to test it some more. Will
post it tomorrow.

> Thanks,
> Suren.
>
> >
> > Thanks,
> > Liam
> >