Re: [PATCH v2 hmm 11/11] mm/hmm: Remove confusing comment and logic from hmm_release

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On 6/6/19 11:44 AM, Jason Gunthorpe wrote:
From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

hmm_release() is called exactly once per hmm. ops->release() cannot
accidentally trigger any action that would recurse back onto
hmm->mirrors_sem.

This fixes a use after-free race of the form:

        CPU0                                   CPU1
                                            hmm_release()
                                              up_write(&hmm->mirrors_sem);
  hmm_mirror_unregister(mirror)
   down_write(&hmm->mirrors_sem);
   up_write(&hmm->mirrors_sem);
   kfree(mirror)
                                              mirror->ops->release(mirror)

The only user we have today for ops->release is an empty function, so this
is unambiguously safe.

As a consequence of plugging this race drivers are not allowed to
register/unregister mirrors from within a release op.

Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

I agree with the analysis above but I'm not sure that release() will
always be an empty function. It might be more efficient to write back
all data migrated to a device "in one pass" instead of relying
on unmap_vmas() calling hmm_start_range_invalidate() per VMA.

I think the bigger issue is potential deadlocks while calling
sync_cpu_device_pagetables() and tasks calling hmm_mirror_unregister():

Say you have three threads:
- Thread A is in try_to_unmap(), either without holding mmap_sem or with
mmap_sem held for read.
- Thread B has some unrelated driver calling hmm_mirror_unregister().
This doesn't require mmap_sem.
- Thread C is about to call migrate_vma().

Thread A                Thread B                 Thread C
try_to_unmap            hmm_mirror_unregister    migrate_vma
----------------------  -----------------------  ----------------------
hmm_invalidate_range_start
down_read(mirrors_sem)
                        down_write(mirrors_sem)
                        // Blocked on A
                                                  device_lock
device_lock
// Blocked on C
                                                  migrate_vma()
                                                  hmm_invalidate_range_s
                                                  down_read(mirrors_sem)
                                                  // Blocked on B
                                                  // Deadlock

Perhaps we should consider using SRCU for walking the mirror->list?

---
  mm/hmm.c | 28 +++++++++-------------------
  1 file changed, 9 insertions(+), 19 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 709d138dd49027..3a45dd3d778248 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -136,26 +136,16 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
  	WARN_ON(!list_empty(&hmm->ranges));
  	mutex_unlock(&hmm->lock);
- down_write(&hmm->mirrors_sem);
-	mirror = list_first_entry_or_null(&hmm->mirrors, struct hmm_mirror,
-					  list);
-	while (mirror) {
-		list_del_init(&mirror->list);
-		if (mirror->ops->release) {
-			/*
-			 * Drop mirrors_sem so the release callback can wait
-			 * on any pending work that might itself trigger a
-			 * mmu_notifier callback and thus would deadlock with
-			 * us.
-			 */
-			up_write(&hmm->mirrors_sem);
+	down_read(&hmm->mirrors_sem);
+	list_for_each_entry(mirror, &hmm->mirrors, list) {
+		/*
+		 * Note: The driver is not allowed to trigger
+		 * hmm_mirror_unregister() from this thread.
+		 */
+		if (mirror->ops->release)
  			mirror->ops->release(mirror);
-			down_write(&hmm->mirrors_sem);
-		}
-		mirror = list_first_entry_or_null(&hmm->mirrors,
-						  struct hmm_mirror, list);
  	}
-	up_write(&hmm->mirrors_sem);
+	up_read(&hmm->mirrors_sem);
hmm_put(hmm);
  }
@@ -287,7 +277,7 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror)
  	struct hmm *hmm = mirror->hmm;
down_write(&hmm->mirrors_sem);
-	list_del_init(&mirror->list);
+	list_del(&mirror->list);
  	up_write(&hmm->mirrors_sem);
  	hmm_put(hmm);
  	memset(&mirror->hmm, POISON_INUSE, sizeof(mirror->hmm));

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