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@xxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxx>
> > Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > 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
