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

[Matthew Wilcox]

On Thu, Jun 27, 2019 at 07:39:37PM -0700, Dan Williams wrote:
> On Thu, Jun 27, 2019 at 12:59 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.

OK, I have a Theory.

get_unlocked_entry() doesn't check the size of the entry being waited for.
So dax_iomap_pmd_fault() can end up sleeping waiting for a PTE entry,
which is (a) foolish, because we know it's going to fall back, and (b)
can lead to a missed wakeup because it's going to sleep waiting for
the PMD entry to come unlocked.  Which it won't, unless there's a happy
accident that happens to map to the same hash bucket.

Let's see if I can steal some time this weekend to whip up a patch.