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

[Suren Baghdasaryan]

On Thu, Aug 10, 2023 at 4:43 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Thu, Aug 10, 2023 at 4:29 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >
> > On Thu, Aug 10, 2023 at 3:16 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.

So far it works without a glitch. Matthew, I think it's fine. If you
post a fixup please add my Tested-by.
Thanks,
Suren.

>
> >
> >
> > >
> > > > 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?