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

[Dan Williams]

On Fri, Jun 28, 2019 at 9:37 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.
> > >
> > > 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.
>
> Okay, good, glad to have ruled that out.
>
> > 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.
>
> That was the conclusion I came to; that one thread holding the mmap sem
> for read isn't being woken up when it should be.  Just need to find it ...
> obviously it's something to do with the PMD entries.

Can you explain to me one more time, yes I'm slow on the uptake on
this, the difference between xas_load() and xas_find_conflict() and
why it's ok for dax_lock_page() to use xas_load() while
grab_mapping_entry() uses xas_find_conflict()?