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

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Message-ID: <f95cd18d-4dda-1992-34ee-7b93f0bbeb9b@redhat.com>
Date:   Fri, 9 Feb 2018 11:49:12 +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 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?

Thanks

Another thing is kvm