Re: [PATCH 1/2] mm/mmu_notifier: init notifier if necessary

[Gavin Shan <shangw@linux.vnet.ibm.com>]

On Fri, Aug 24, 2012 at 02:51:51PM -0700, Andrew Morton wrote:
>On Fri, 24 Aug 2012 22:37:55 +0800
>Wanpeng Li <liwanp@linux.vnet.ibm.com> wrote:
>
>> From: Gavin Shan <shangw@linux.vnet.ibm.com>
>> 
>> 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 <shangw@linux.vnet.ibm.com>
>> Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
>> ---
>>  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

>> +		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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