[dpdk-dev] [RFC] pci: force address of mappings in secondary process

Sergio Gonzalez Monroy sergio.gonzalez.monroy at intel.com
Tue Jul 11 13:35:39 CEST 2017


On 11/07/2017 02:56, Tan, Jianfeng wrote:
>> -----Original Message-----
>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Stephen
>> Hemminger
>> Sent: Tuesday, July 11, 2017 9:13 AM
>> To: dev at dpdk.org
>> Cc: Stephen Hemminger
>> Subject: [dpdk-dev] [RFC] pci: force address of mappings in secondary
>> process
>>
>> The PCI memory resources in the secondary process should be in
>> the exact same location as the primary process. Otherwise
>> there is a risk of a stray pointer.
>>
>> Not sure if this is right, but it looks like a potential
>> problem.
>>
>> ---
>>   lib/librte_eal/common/eal_common_pci_uio.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/librte_eal/common/eal_common_pci_uio.c
>> b/lib/librte_eal/common/eal_common_pci_uio.c
>> index 367a6816dcb8..2156b1a436c4 100644
>> --- a/lib/librte_eal/common/eal_common_pci_uio.c
>> +++ b/lib/librte_eal/common/eal_common_pci_uio.c
>> @@ -77,7 +77,7 @@ pci_uio_map_secondary(struct rte_pci_device *dev)
>>
>>   			void *mapaddr = pci_map_resource(uio_res-
>>> maps[i].addr,
>>   					fd, (off_t)uio_res->maps[i].offset,
>> -					(size_t)uio_res->maps[i].size, 0);
>> +					(size_t)uio_res->maps[i].size,
>> MAP_FIXED);
>>   			/* fd is not needed in slave process, close it */
>>   			close(fd);
>>   			if (mapaddr != uio_res->maps[i].addr) {
>> --
>> 2.11.0
> +1 for this RFC. I also once encounter such problem, and I use the same way to solve it. The addr parameter of mmap() syscall is only a hint instead of a must even the VMA is not occupied yet.
>
> Thanks,
> Jianfeng

How do you know the VMA is not occupied?

I think the risk here is that the dynamic linker loaded some shared 
library in that VMA, and forcing MAP_FIXED is not a safe solution.
What I have observed is that Linux will return a different VMA than the 
one hinted when there is already a mapping in the requested/hinted VMA.

I reckon this is a similar issue as we have with the multi-process model 
when we do not get the VMA requested for the huge-pages.
AFAIK we do not have a robust solution for this issue other than restart 
the program and hope the dynamic linker does not map anything in the VMA 
ranges that we need to map from the primary. This is also assuming that 
the application does not allocate memory and maps things before calling 
eal_init as it could potentially use VMA ranges that we need in the 
secondary process.

The proposal for new secondary process model would solve these issues:
http://dpdk.org/ml/archives/dev/2017-May/066147.html

Thanks,
Sergio


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