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

[Dan Williams]

On Sat, Jun 29, 2019 at 9:03 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> 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.

Theory seems to have some evidence... I instrumented fs/dax.c to track
outstanding 'lock' entries and 'wait' events. At the time of the hang
we see no locks held and the waiter is waiting on a pmd entry:

[ 4001.354334] fs/dax locked entries: 0
[ 4001.358425] fs/dax wait entries: 1
[ 4001.362227] db_bench/2445 index: 0x0 shift: 6
[ 4001.367099]  grab_mapping_entry+0x17a/0x260
[ 4001.371773]  dax_iomap_pmd_fault.isra.43+0x168/0x7a0
[ 4001.377316]  ext4_dax_huge_fault+0x16f/0x1f0
[ 4001.382086]  __handle_mm_fault+0x411/0x1390
[ 4001.386756]  handle_mm_fault+0x172/0x360