netdev - Re: [PATCH net V3 1/2] ptr_ring: try vmalloc() when kmalloc() fails

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Date:   Thu, 8 Feb 2018 14:58:40 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net V3 1/2] ptr_ring: try vmalloc() when kmalloc() fails



On 2018年02月08日 12:45, Michael S. Tsirkin wrote:
> On Thu, Feb 08, 2018 at 11:59:24AM +0800, Jason Wang wrote:
>> This patch switch to use kvmalloc_array() for using a vmalloc()
>> fallback to help in case kmalloc() fails.
>>
>> Reported-by: syzbot+e4d4f9ddd4295539735d@...kaller.appspotmail.com
>> Fixes: 2e0ab8ca83c12 ("ptr_ring: array based FIFO for pointers")
> I guess the actual patch is the one that switches tun to ptr_ring.

I think not, since the issue was large allocation.

>
> In fact, I think the actual bugfix is patch 2/2. This specific one
> just makes kmalloc less likely to fail but that's
> not what syzbot reported.

Agree.

>
> Then I would add this patch on top to make kmalloc less likely to fail.

Ok.

>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>
>
>> ---
>>   include/linux/ptr_ring.h | 10 +++++-----
>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
>> index 1883d61..2af71a7 100644
>> --- a/include/linux/ptr_ring.h
>> +++ b/include/linux/ptr_ring.h
>> @@ -466,7 +466,7 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
>>   
>>   static inline void **__ptr_ring_init_queue_alloc(unsigned int size, gfp_t gfp)
>>   {
>> -	return kcalloc(size, sizeof(void *), gfp);
>> +	return kvmalloc_array(size, sizeof(void *), gfp | __GFP_ZERO);
>>   }
>>   
>>   static inline void __ptr_ring_set_size(struct ptr_ring *r, int size)
> This implies a bunch of limitations on the flags. From kvmalloc_node
> docs:
>
>   * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not supported.
>   * __GFP_RETRY_MAYFAIL is supported, and it should be used only if kmalloc is
>   * preferable to the vmalloc fallback, due to visible performance drawbacks.
>
> Fine with all the current users, but if we go this way, please add
> documentation so future users don't misuse this API.

I suspect this is somehow a overkill since this means we need sync with 
mm/vmalloc changes in the future to keep it synced.

> Alternatively, test flags and call kvmalloc or kcalloc?

Similar to the above issue, I would rather leave it as is.

Thanks

>
>
>> @@ -601,7 +601,7 @@ static inline int ptr_ring_resize(struct ptr_ring *r, int size, gfp_t gfp,
>>   	spin_unlock(&(r)->producer_lock);
>>   	spin_unlock_irqrestore(&(r)->consumer_lock, flags);
>>   
>> -	kfree(old);
>> +	kvfree(old);
>>   
>>   	return 0;
>>   }
>> @@ -641,7 +641,7 @@ static inline int ptr_ring_resize_multiple(struct ptr_ring **rings,
>>   	}
>>   
>>   	for (i = 0; i < nrings; ++i)
>> -		kfree(queues[i]);
>> +		kvfree(queues[i]);
>>   
>>   	kfree(queues);
>>   
>> @@ -649,7 +649,7 @@ static inline int ptr_ring_resize_multiple(struct ptr_ring **rings,
>>   
>>   nomem:
>>   	while (--i >= 0)
>> -		kfree(queues[i]);
>> +		kvfree(queues[i]);
>>   
>>   	kfree(queues);
>>   
>> @@ -664,7 +664,7 @@ static inline void ptr_ring_cleanup(struct ptr_ring *r, void (*destroy)(void *))
>>   	if (destroy)
>>   		while ((ptr = ptr_ring_consume(r)))
>>   			destroy(ptr);
>> -	kfree(r->queue);
>> +	kvfree(r->queue);
>>   }
>>   
>>   #endif /* _LINUX_PTR_RING_H  */
>> -- 
>> 2.7.4