On Thu, Aug 30, 2012 at 12:13:02PM -0700, Andrew Morton wrote:

On Sat, 25 Aug 2012 17:47:50 +0800
Gavin Shan [off-list ref] wrote:

quoted

quoted

quoted

--- 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 :-)

GFP_ATOMIC allocations are unreliable.  If the allocation attempt fails
here, an entire kernel subsystem will have failed, quite probably
requiring a reboot.  It's a bad tradeoff.

Yep. Thanks, Andrew :-)

Please fix this and retest.  With lockdep enabled, of course.

And please do not attempt to sneak changes like this into the kernel
without even mentioning them in the changelog.  If I hadn't have
happened to notice this, we'd have ended up with a less reliable
kernel.

I'll fix and rerest it.

Thanks,
Gavin

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