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

zhangfei.gao
2022-04-16T02:04:54Z

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;

default:
// master process directly exit, and release mm as well as ioasid
exit(0);
}

// only daemon process

Thanks

>
> Thanks,
>
> Jacob