Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

zhangfei.gao
2022-04-19T01:03:06Z

On 2022/4/19 上午2:14, Jacob Pan wrote:
> Hi zhangfei.gao@foxmail.com,
>
> On Sat, 16 Apr 2022 09:43:07 +0800, "zhangfei.gao@foxmail.com"
> <zhangfei.gao@foxmail.com> wrote:
>
>> On 2022/4/16 上午5:00, Jacob Pan wrote:
>>> Hi zhangfei.gao@foxmail.com,
>>>
>>> On Fri, 15 Apr 2022 19:52:03 +0800, "zhangfei.gao@foxmail.com"
>>> <zhangfei.gao@foxmail.com> wrote:
>>>
>>>>>>> A PASID might be still used even though it is freed on mm exit.
>>>>>>>
>>>>>>> process A:
>>>>>>> sva_bind();
>>>>>>> ioasid_alloc() = N; // Get PASID N for the mm
>>>>>>> fork(): // spawn process B
>>>>>>> exit();
>>>>>>> ioasid_free(N);
>>>>>>>
>>>>>>> process B:
>>>>>>> device uses PASID N -> failure
>>>>>>> sva_unbind();
>>>>>>>
>>>>>>> Dave Hansen suggests to take a refcount on the mm whenever binding
>>>>>>> the PASID to a device and drop the refcount on unbinding. The mm
>>>>>>> won't be dropped if the PASID is still bound to it.
>>>>>>>
>>>>>>> Fixes: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID
>>>>>>> allocation and free it on mm exit")
>>>>>>>
>>> Is process A's mm intended to be used by process B? Or you really should
>>> use PASID N on process B's mm? If the latter, it may work for a while
>>> until B changes mapping.
>>>
>>> It seems you are just extending the life of a defunct mm?
>> From nginx code, the master process init resources, then fork daemon
>> process to take over,
>> then master process exit by itself.
>>
>> src/core/nginx.c
>> main
>> ngx_ssl_init(log);    -> openssl engine -> bind_fn -> sva_bind()
>> ngx_daemon(cycle->log)
>>
>> src/os/unix/ngx_daemon.c
>> ngx_daemon(ngx_log_t *log)
>> {
>>      int  fd;
>>
>>      switch (fork()) {
>>      case -1:
>>          ngx_log_error(NGX_LOG_EMERG, log, ngx_errno, "fork() failed");
>>          return NGX_ERROR;
>>
>>      case 0:
>>         // the fork daemon process
>>          break;
>>
> Does this child process call sva_bind() again to get another PASID? Or it
> will keep using the parent's PASID for DMA?
The master process call sva_bind (PASID A), fork daemon process, then exit.

The daemon process does not call sva_bind again, only for managing
worker processes.

The worker process will call sva_bind for new PASID (B), for real
transaction.

The worker process will free the PASID (B) when worker process exit like
nginx quit.

nginx -s quit does not free PASID A via callback, which may should be
freed by signal handler in engine itself, still in check.

Thanks