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SubjectRe: [PATCH] filesystem-dax: Disable PMD support
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@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.

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.

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