Re: [PATCH v2 hmm 05/11] mm/hmm: Remove duplicate condition test before wait_event_timeout

[Jason Gunthorpe <jgg-uk2M96/98Pc@public.gmane.org>]

On Fri, Jun 07, 2019 at 12:01:45PM -0700, Ralph Campbell wrote:
> 
> On 6/6/19 11:44 AM, Jason Gunthorpe wrote:
> > From: Jason Gunthorpe <jgg@mellanox.com>
> > 
> > The wait_event_timeout macro already tests the condition as its first
> > action, so there is no reason to open code another version of this, all
> > that does is skip the might_sleep() debugging in common cases, which is
> > not helpful.
> > 
> > Further, based on prior patches, we can no simplify the required condition
> > test:
> >   - If range is valid memory then so is range->hmm
> >   - If hmm_release() has run then range->valid is set to false
> >     at the same time as dead, so no reason to check both.
> >   - A valid hmm has a valid hmm->mm.
> > 
> > Also, add the READ_ONCE for range->valid as there is no lock held here.
> > 
> > Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> > Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> >   include/linux/hmm.h | 12 ++----------
> >   1 file changed, 2 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> > index 4ee3acabe5ed22..2ab35b40992b24 100644
> > +++ b/include/linux/hmm.h
> > @@ -218,17 +218,9 @@ static inline unsigned long hmm_range_page_size(const struct hmm_range *range)
> >   static inline bool hmm_range_wait_until_valid(struct hmm_range *range,
> >   					      unsigned long timeout)
> >   {
> > -	/* Check if mm is dead ? */
> > -	if (range->hmm == NULL || range->hmm->dead || range->hmm->mm == NULL) {
> > -		range->valid = false;
> > -		return false;
> > -	}
> > -	if (range->valid)
> > -		return true;
> > -	wait_event_timeout(range->hmm->wq, range->valid || range->hmm->dead,
> > +	wait_event_timeout(range->hmm->wq, range->valid,
> >   			   msecs_to_jiffies(timeout));
> > -	/* Return current valid status just in case we get lucky */
> > -	return range->valid;
> > +	return READ_ONCE(range->valid);
> >   }
> >   /*
> > 
> 
> Since we are simplifying things, perhaps we should consider merging
> hmm_range_wait_until_valid() info hmm_range_register() and
> removing hmm_range_wait_until_valid() since the pattern
> is to always call the two together.

? the hmm.rst shows the hmm_range_wait_until_valid being called in the
(ret == -EAGAIN) path. It is confusing because it should really just
have the again label moved up above hmm_range_wait_until_valid() as
even if we get the driver lock it could still be a long wait for the
colliding invalidation to clear.

What I want to get to is a pattern like this:

pagefault():

   hmm_range_register(&range);
again:
   /* On the slow path, if we appear to be live locked then we get
      the write side of mmap_sem which will break the live lock,
      otherwise this gets the read lock */
   if (hmm_range_start_and_lock(&range))
         goto err;

   lockdep_assert_held(range->mm->mmap_sem);

   // Optional: Avoid useless expensive work
   if (hmm_range_needs_retry(&range))
      goto again;
   hmm_range_(touch vmas)

   take_lock(driver->update);
   if (hmm_range_end(&range) {
       release_lock(driver->update);
       goto again;
   }
   // Finish driver updates
   release_lock(driver->update);

   // Releases mmap_sem
   hmm_range_unregister_and_unlock(&range);

What do you think? 

Is it clear?

Jason
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