On Fri, Aug 24, 2012 at 02:51:51PM -0700, Andrew Morton wrote:

On Fri, 24 Aug 2012 22:37:55 +0800
Wanpeng Li [off-list ref] wrote:

quoted

From: Gavin Shan <redacted>

While registering MMU notifier, new instance of MMU notifier_mm will
be allocated and later free'd if currrent mm_struct's MMU notifier_mm
has been initialized. That cause some overhead. The patch tries to
eleminate that.

Signed-off-by: Gavin Shan <redacted>
Signed-off-by: Wanpeng Li <redacted>
---
 mm/mmu_notifier.c |   22 +++++++++++-----------
 1 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index 862b608..fb4067f 100644
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c

@@ -192,22 +192,23 @@ static int do_mmu_notifier_register(struct mmu_notifier *mn,
 
 	BUG_ON(atomic_read(&mm->mm_users) <= 0);
 
-	ret = -ENOMEM;
-	mmu_notifier_mm = kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
-	if (unlikely(!mmu_notifier_mm))
-		goto out;
-
 	if (take_mmap_sem)
 		down_write(&mm->mmap_sem);
 	ret = mm_take_all_locks(mm);
 	if (unlikely(ret))
-		goto out_cleanup;
+		goto out;
 
 	if (!mm_has_notifiers(mm)) {
+		mmu_notifier_mm = kmalloc(sizeof(struct mmu_notifier_mm),
+					GFP_ATOMIC);

Why was the code switched to the far weaker GFP_ATOMIC?  We can still
perform sleeping allocations inside mmap_sem.

Yes, we can perform sleeping while allocating memory, but we're holding
the "mmap_sem". GFP_KERNEL possiblly block somebody else who also waits
on mmap_sem for long time even though the case should be rare :-)

Thanks,
Gavin

quoted

+		if (unlikely(!mmu_notifier_mm)) {
+			ret = -ENOMEM;
+			goto out_of_mem;
+		}
 		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;
 	}
 	atomic_inc(&mm->mm_count);
 

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