linux-kernel - Re: [PATCH] dax: Fix missed PMD wakeups

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Date:   Wed, 3 Jul 2019 20:27:28 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Boaz Harrosh <openosd@...il.com>,
        stable <stable@...r.kernel.org>,
        Robert Barror <robert.barror@...el.com>,
        Seema Pandit <seema.pandit@...el.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dax: Fix missed PMD wakeups

On Wed, Jul 03, 2019 at 02:28:41PM -0700, Dan Williams wrote:
> On Wed, Jul 3, 2019 at 12:53 PM Matthew Wilcox <willy@...radead.org> wrote:
> > @@ -211,7 +215,8 @@ static void *get_unlocked_entry(struct xa_state *xas)
> >         for (;;) {
> >                 entry = xas_find_conflict(xas);
> >                 if (!entry || WARN_ON_ONCE(!xa_is_value(entry)) ||
> > -                               !dax_is_locked(entry))
> > +                               !dax_is_locked(entry) ||
> > +                               dax_entry_order(entry) < xas_get_order(xas))
> 
> Doesn't this potentially allow a locked entry to be returned for a
> caller that expects all value entries are unlocked?

It only allows locked entries to be returned for callers which pass in
an xas which refers to a PMD entry.  This is fine for grab_mapping_entry()
because it checks size_flag & is_pte_entry.

dax_layout_busy_page() only uses 0-order.
__dax_invalidate_entry() only uses 0-order.
dax_writeback_one() needs an extra fix:

                /* Did a PMD entry get split? */
                if (dax_is_locked(entry))
                        goto put_unlocked;

dax_insert_pfn_mkwrite() checks for a mismatch of pte vs pmd.

So I think we're good for all current users.

> > +#ifdef CONFIG_XARRAY_MULTI
> > +       unsigned int sibs = xas->xa_sibs;
> > +
> > +       while (sibs) {
> > +               order++;
> > +               sibs /= 2;
> > +       }
> 
> Use ilog2() here?

Thought about it.  sibs is never going to be more than 31, so I don't
know that it's worth eliminating 5 add/shift pairs in favour of whatever
the ilog2 instruction is on a given CPU.  In practice, on x86, sibs is
going to be either 0 (PTEs) or 7 (PMDs).  We could also avoid even having
this function by passing PMD_ORDER or PTE_ORDER into get_unlocked_entry().

It's probably never going to be noticable in this scenario because it's
the very last thing checked before we put ourselves on a waitqueue and
go to sleep.