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

Jacob Pan
2022-04-18T18:11:27Z

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?

>      default:
>        // master process directly exit, and release mm as well as ioasid
>          exit(0);
>      }
>
>       // only daemon process
>
> Thanks
>
> >
> > Thanks,
> >
> > Jacob
>

Thanks,

Jacob