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

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Date:   Wed, 10 Jul 2019 21:02:04 +0200
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        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 Fri 05-07-19 13:47:02, Dan Williams wrote:
> On Fri, Jul 5, 2019 at 12:10 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Thu, Jul 04, 2019 at 04:27:14PM -0700, Dan Williams wrote:
> > > On Thu, Jul 4, 2019 at 12:14 PM Matthew Wilcox <willy@...radead.org> wrote:
> > > >
> > > > On Thu, Jul 04, 2019 at 06:54:50PM +0200, Jan Kara wrote:
> > > > > On Wed 03-07-19 20:27:28, Matthew Wilcox wrote:
> > > > > > So I think we're good for all current users.
> > > > >
> > > > > Agreed but it is an ugly trap. As I already said, I'd rather pay the
> > > > > unnecessary cost of waiting for pte entry and have an easy to understand
> > > > > interface. If we ever have a real world use case that would care for this
> > > > > optimization, we will need to refactor functions to make this possible and
> > > > > still keep the interfaces sane. For example get_unlocked_entry() could
> > > > > return special "error code" indicating that there's no entry with matching
> > > > > order in xarray but there's a conflict with it. That would be much less
> > > > > error-prone interface.
> > > >
> > > > This is an internal interface.  I think it's already a pretty gnarly
> > > > interface to use by definition -- it's going to sleep and might return
> > > > almost anything.  There's not much scope for returning an error indicator
> > > > either; value entries occupy half of the range (all odd numbers between 1
> > > > and ULONG_MAX inclusive), plus NULL.  We could use an internal entry, but
> > > > I don't think that makes the interface any easier to use than returning
> > > > a locked entry.
> > > >
> > > > I think this iteration of the patch makes it a little clearer.  What do you
> > > > think?
> > > >
> > >
> > > Not much clearer to me. get_unlocked_entry() is now misnamed and this
> >
> > misnamed?  You'd rather it was called "try_get_unlocked_entry()"?
> 
> I was thinking more along the lines of
> get_unlocked_but_sometimes_locked_entry(), i.e. per Jan's feedback to
> keep the interface simple.

So how about the attached patch? That keeps the interface sane and passes a
smoketest for me (full fstest run running). Obviously it also needs a
proper changelog...

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-dax-Fix-missed-PMD-wakeups.patch" of type "text/x-patch" (5442 bytes)