On 6/7/19 12:13 PM, Jason Gunthorpe wrote:
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
Are you talking about acquiring mmap_sem in hmm_range_start_and_lock()?
Usually, the fault code has to lock mmap_sem for read in order to
call find_vma() so it can set range.vma.
If HMM drops mmap_sem - which I don't think it should, just return an
error to tell the caller to drop mmap_sem and retry - the find_vma()
will need to be repeated as well.
I'm also not sure about acquiring the mmap_sem for write as way to
mitigate thrashing. It seems to me that if a device and a CPU are
both faulting on the same page, some sort of backoff delay is needed
to let one side or the other make some progress.
Thrashing mitigation and how migrate_vma() plays in this is a
deep topic for thought.
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