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Subject[PATCH v3 hmm 02/11] mm/mmu_notifiers: do not speculatively allocate a mmu_notifier_mm
Date
From: Jason Gunthorpe <jgg@mellanox.com>

A prior commit e0f3c3f78da2 ("mm/mmu_notifier: init notifier if necessary")
made an attempt at doing this, but had to be reverted as calling
the GFP_KERNEL allocator under the i_mmap_mutex causes deadlock, see
commit 35cfa2b0b491 ("mm/mmu_notifier: allocate mmu_notifier in advance").

However, we can avoid that problem by doing the allocation only under
the mmap_sem, which is already happening.

Since all writers to mm->mmu_notifier_mm hold the write side of the
mmap_sem reading it under that sem is deterministic and we can use that to
decide if the allocation path is required, without speculation.

The actual update to mmu_notifier_mm must still be done under the
mm_take_all_locks() to ensure read-side coherency.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
---
mm/mmu_notifier.c | 34 ++++++++++++++++++++++------------
1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index 218a6f108bc2d0..696810f632ade1 100644
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -242,27 +242,32 @@ EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range);
*/
int __mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
{
- struct mmu_notifier_mm *mmu_notifier_mm;
+ struct mmu_notifier_mm *mmu_notifier_mm = NULL;
int ret;

lockdep_assert_held_write(&mm->mmap_sem);
BUG_ON(atomic_read(&mm->mm_users) <= 0);

- mmu_notifier_mm = kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
- if (unlikely(!mmu_notifier_mm))
- return -ENOMEM;
+ if (!mm->mmu_notifier_mm) {
+ /*
+ * kmalloc cannot be called under mm_take_all_locks(), but we
+ * know that mm->mmu_notifier_mm can't change while we hold
+ * the write side of the mmap_sem.
+ */
+ mmu_notifier_mm =
+ kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
+ if (!mmu_notifier_mm)
+ return -ENOMEM;
+
+ INIT_HLIST_HEAD(&mmu_notifier_mm->list);
+ spin_lock_init(&mmu_notifier_mm->lock);
+ }

ret = mm_take_all_locks(mm);
if (unlikely(ret))
goto out_clean;

- if (!mm_has_notifiers(mm)) {
- INIT_HLIST_HEAD(&mmu_notifier_mm->list);
- spin_lock_init(&mmu_notifier_mm->lock);
-
- mm->mmu_notifier_mm = mmu_notifier_mm;
- mmu_notifier_mm = NULL;
- }
+ /* Pairs with the mmdrop in mmu_notifier_unregister_* */
mmgrab(mm);

/*
@@ -273,14 +278,19 @@ int __mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm)
* We can't race against any other mmu notifier method either
* thanks to mm_take_all_locks().
*/
+ if (mmu_notifier_mm)
+ mm->mmu_notifier_mm = mmu_notifier_mm;
+
spin_lock(&mm->mmu_notifier_mm->lock);
hlist_add_head_rcu(&mn->hlist, &mm->mmu_notifier_mm->list);
spin_unlock(&mm->mmu_notifier_mm->lock);

mm_drop_all_locks(mm);
+ BUG_ON(atomic_read(&mm->mm_users) <= 0);
+ return 0;
+
out_clean:
kfree(mmu_notifier_mm);
- BUG_ON(atomic_read(&mm->mm_users) <= 0);
return ret;
}
EXPORT_SYMBOL_GPL(__mmu_notifier_register);
--
2.22.0
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