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

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Date:   Fri, 9 Feb 2018 12:04:53 +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09日 11:56, Michael S. Tsirkin wrote:
> On Fri, Feb 09, 2018 at 11:49:12AM +0800, Jason Wang wrote:
>>
>> On 2018年02月09日 03:17, Michael S. Tsirkin wrote:
>>> On Thu, Feb 08, 2018 at 02:58:40PM +0800, Jason Wang wrote:
>>>> 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
>>> How do we prevent someone from inevitably trying to use this with
>>> GFP_ATOMIC?
>>>
>> Well, we somehow can't prevent this even if there's a documentation, that's
>> why there's a BUG() in vmalloc code I think. And kvmalloc also requires
>> GFP_KERNEL otherewise another WARN().
>>
>> So looks like the WARN()/BUG() should be sufficient?
> Well vmalloc only triggers when you pass in a huge size.
> Let's settle for

There's a:

     BUG_ON(in_interrupt());

in __get_vm_area_node().

>
> /* Not all gfp_t flags (besides GFP_KERNEL) are allowed. See
>   * documentation for vmalloc for which of them are legal.
>   */

Fine with me.

>> Thanks
>>
>> Another thing is kvm
> ?

Sorry typo.

Thanks