Re: [PATCH v7 0/6] Per-VMA lock support for swap and userfaults

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Aug 10, 2023 at 4:29 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Thu, Aug 10, 2023 at 3:16 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Thu, Aug 10, 2023 at 06:24:15AM +0000, Suren Baghdasaryan wrote:
> > > Ok, I think I found the issue.  wp_page_shared() ->
> > > fault_dirty_shared_page() can drop mmap_lock (see the comment saying
> > > "Drop the mmap_lock before waiting on IO, if we can...", therefore we
> > > have to ensure we are not doing this under per-VMA lock.
> >
> > ... or we could change maybe_unlock_mmap_for_io() the same way
> > that we changed folio_lock_or_retry():
> >
> > +++ b/mm/internal.h
> > @@ -706,7 +706,7 @@ static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
> >         if (fault_flag_allow_retry_first(flags) &&
> >             !(flags & FAULT_FLAG_RETRY_NOWAIT)) {
> >                 fpin = get_file(vmf->vma->vm_file);
> > -               mmap_read_unlock(vmf->vma->vm_mm);
> > +               release_fault_lock(vmf);
> >         }
> >         return fpin;
> >  }
> >
> > What do you think?
>
> This is very tempting... Let me try that and see if anything explodes,
> but yes, this would be ideal.

Ok, so far looks good, the problem is not reproducible. I'll run some
more exhaustive testing today.

>
>
> >
> > > I think what happens is that this path is racing with another page
> > > fault which took mmap_lock for read. fault_dirty_shared_page()
> > > releases this lock which was taken by another page faulting thread and
> > > that thread generates an assertion when it finds out the lock it just
> > > took got released from under it.
> >
> > I'm confused that our debugging didn't catch this earlier.  lockdep
> > should always catch this.
>
> Maybe this condition is rare enough?