[dpdk-dev] [PATCH v2] eal/linux: do not create user mem map repeatedly when it exists

[Burakov, Anatoly]

On 25-Jul-20 10:59 AM, wangyunjian wrote:
>> -----Original Message-----
>> From: Burakov, Anatoly [mailto:anatoly.burakov at intel.com]
>> Sent: Friday, July 24, 2020 9:25 PM
>> To: wangyunjian <wangyunjian at huawei.com>; dev at dpdk.org;
>> david.marchand at redhat.com
>> Cc: Lilijun (Jerry) <jerry.lilijun at huawei.com>; xudingke
>> <xudingke at huawei.com>; stable at dpdk.org
>> Subject: Re: [dpdk-dev] [PATCH v2] eal/linux: do not create user mem map
>> repeatedly when it exists
>>
>> On 23-Jul-20 3:48 PM, wangyunjian wrote:
>>> From: Yunjian Wang <wangyunjian at huawei.com>
>>>
>>> Currently, we will create new user mem map entry for the same memory
>>> segment, but in fact it has already been added to the user mem maps.
>>> It's not necessary to create it twice.
>>>
>>> To resolve the issue, add support to remove the same entry in the
>>> function compact_user_maps().
>>>
>>> Fixes: 0cbce3a167f1 ("vfio: skip DMA map failure if already mapped")
>>> Cc: stable at dpdk.org
>>>
>>> Signed-off-by: Yunjian Wang <wangyunjian at huawei.com>
>>> ---
>>> v2:
>>> * Remove the same entry in the function compact_user_maps()
>>> ---
>>>    lib/librte_eal/linux/eal_vfio.c | 5 +++++
>>>    1 file changed, 5 insertions(+)
>>>
>>> diff --git a/lib/librte_eal/linux/eal_vfio.c b/lib/librte_eal/linux/eal_vfio.c
>>> index abb12a354..df99307b7 100644
>>> --- a/lib/librte_eal/linux/eal_vfio.c
>>> +++ b/lib/librte_eal/linux/eal_vfio.c
>>> @@ -167,6 +167,10 @@ adjust_map(struct user_mem_map *src, struct
>> user_mem_map *end,
>>>    static int
>>>    merge_map(struct user_mem_map *left, struct user_mem_map *right)
>>>    {
>>> +	/* merge the same maps into one */
>>> +	if (memcmp(left, right, sizeof(struct user_mem_map)) == 0)
>>> +		goto out;
>>> +
>>
>> merge_map looks for adjacent maps only, but does not handle maps that
>> are wholly contained within one another ("the same map" also matches
>> this definition). wouldn't it be better to check for that instead of
>> *just* handling identical maps?
> 
> What about using the initial implementation?
> We don't create new user mem map entry for the same memory segment.

I don't like this implementation because it relies on particulars of how 
VFIO mapping work without explicitly specifying them. I.e. it's prone to 
breaking when changing code. That's not even mentioning that we have no 
guarantees on kernel behavior in that particular case being identical on 
all supported platforms.

I would honestly prefer an explicit compaction over implicit one.

> 
> @@ -1828,6 +1828,13 @@  container_dma_map(struct vfio_config *vfio_cfg, uint64_t vaddr, uint64_t iova,
>   		ret = -1;
>   		goto out;
>   	}
> +
> +	/* we don't need create new user mem map entry
> +	 * for the same memory segment.
> +	 */
> +	if (errno == EBUSY || errno == EEXIST)
> +		goto out;
> +
>   	/* create new user mem map entry */
>   	new_map = &user_mem_maps->maps[user_mem_maps->n_maps++];
>   	new_map->addr = vaddr;
> 
> Thanks,
> Yunjian
>>
>>>    	if (left->addr + left->len != right->addr)
>>>    		return 0;
>>>    	if (left->iova + left->len != right->iova)
>>> @@ -174,6 +178,7 @@ merge_map(struct user_mem_map *left, struct
>> user_mem_map *right)
>>>
>>>    	left->len += right->len;
>>>
>>> +out:
>>>    	memset(right, 0, sizeof(*right));
>>>
>>>    	return 1;
>>>
>>
>>
>> --
>> Thanks,
>> Anatoly


-- 
Thanks,
Anatoly