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

[Matthew Wilcox <willy@infradead.org>]

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?