Re: [PATCH] filesystem-dax: Disable PMD support

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Jun 27, 2019 at 12:59 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, Jun 27, 2019 at 12:09:29PM -0700, Dan Williams wrote:
> > > This bug feels like we failed to unlock, or unlocked the wrong entry
> > > and this hunk in the bisected commit looks suspect to me. Why do we
> > > still need to drop the lock now that the radix_tree_preload() calls
> > > are gone?
> >
> > Nevermind, unmapp_mapping_pages() takes a sleeping lock, but then I
> > wonder why we don't restart the lookup like the old implementation.
>
> We have the entry locked:
>
>                 /*
>                  * Make sure 'entry' remains valid while we drop
>                  * the i_pages lock.
>                  */
>                 dax_lock_entry(xas, entry);
>
>                 /*
>                  * Besides huge zero pages the only other thing that gets
>                  * downgraded are empty entries which don't need to be
>                  * unmapped.
>                  */
>                 if (dax_is_zero_entry(entry)) {
>                         xas_unlock_irq(xas);
>                         unmap_mapping_pages(mapping,
>                                         xas->xa_index & ~PG_PMD_COLOUR,
>                                         PG_PMD_NR, false);
>                         xas_reset(xas);
>                         xas_lock_irq(xas);
>                 }
>
> If something can remove a locked entry, then that would seem like the
> real bug.  Might be worth inserting a lookup there to make sure that it
> hasn't happened, I suppose?

Nope, added a check, we do in fact get the same locked entry back
after dropping the lock.

The deadlock revolves around the mmap_sem. One thread holds it for
read and then gets stuck indefinitely in get_unlocked_entry(). Once
that happens another rocksdb thread tries to mmap and gets stuck
trying to take the mmap_sem for write. Then all new readers, including
ps and top that try to access a remote vma, then get queued behind
that write.

It could also be the case that we're missing a wake up.