Re: [PATCH v2 hmm 03/11] mm/hmm: Hold a mmgrab from hmm to mm

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

So long a a struct hmm pointer exists, so should the struct mm it is

s/a a/as a/

linked too. Hold the mmgrab() as soon as a hmm is created, and mmdrop() it
once the hmm refcount goes to zero.

Since mmdrop() (ie a 0 kref on struct mm) is now impossible with a !NULL
mm->hmm delete the hmm_hmm_destroy().

Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>

Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>

---
v2:
  - Fix error unwind paths in hmm_get_or_create (Jerome/Jason)
---
  include/linux/hmm.h |  3 ---
  kernel/fork.c       |  1 -
  mm/hmm.c            | 22 ++++------------------
  3 files changed, 4 insertions(+), 22 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 2d519797cb134a..4ee3acabe5ed22 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -586,14 +586,11 @@ static inline int hmm_vma_fault(struct hmm_mirror *mirror,
  }
/* Below are for HMM internal use only! Not to be used by device driver! */
-void hmm_mm_destroy(struct mm_struct *mm);
-
  static inline void hmm_mm_init(struct mm_struct *mm)
  {
  	mm->hmm = NULL;
  }
  #else /* IS_ENABLED(CONFIG_HMM_MIRROR) */
-static inline void hmm_mm_destroy(struct mm_struct *mm) {}
  static inline void hmm_mm_init(struct mm_struct *mm) {}
  #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
diff --git a/kernel/fork.c b/kernel/fork.c
index b2b87d450b80b5..588c768ae72451 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -673,7 +673,6 @@ void __mmdrop(struct mm_struct *mm)
  	WARN_ON_ONCE(mm == current->active_mm);
  	mm_free_pgd(mm);
  	destroy_context(mm);
-	hmm_mm_destroy(mm);
  	mmu_notifier_mm_destroy(mm);
  	check_mm(mm);
  	put_user_ns(mm->user_ns);
diff --git a/mm/hmm.c b/mm/hmm.c
index 8796447299023c..cc7c26fda3300e 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -29,6 +29,7 @@
  #include <linux/swapops.h>
  #include <linux/hugetlb.h>
  #include <linux/memremap.h>
+#include <linux/sched/mm.h>
  #include <linux/jump_label.h>
  #include <linux/dma-mapping.h>
  #include <linux/mmu_notifier.h>
@@ -82,6 +83,7 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
  	hmm->notifiers = 0;
  	hmm->dead = false;
  	hmm->mm = mm;
+	mmgrab(hmm->mm);
spin_lock(&mm->page_table_lock);
  	if (!mm->hmm)
@@ -109,6 +111,7 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
  		mm->hmm = NULL;
  	spin_unlock(&mm->page_table_lock);
  error:
+	mmdrop(hmm->mm);
  	kfree(hmm);
  	return NULL;
  }
@@ -130,6 +133,7 @@ static void hmm_free(struct kref *kref)
  		mm->hmm = NULL;
  	spin_unlock(&mm->page_table_lock);
+ mmdrop(hmm->mm);
  	mmu_notifier_call_srcu(&hmm->rcu, hmm_free_rcu);
  }
@@ -138,24 +142,6 @@ static inline void hmm_put(struct hmm *hmm)
  	kref_put(&hmm->kref, hmm_free);
  }
-void hmm_mm_destroy(struct mm_struct *mm)
-{
-	struct hmm *hmm;
-
-	spin_lock(&mm->page_table_lock);
-	hmm = mm_get_hmm(mm);
-	mm->hmm = NULL;
-	if (hmm) {
-		hmm->mm = NULL;
-		hmm->dead = true;
-		spin_unlock(&mm->page_table_lock);
-		hmm_put(hmm);
-		return;
-	}
-
-	spin_unlock(&mm->page_table_lock);
-}
-
  static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
  {
  	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);

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