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

[zhangfei.gao@xxxxxxxxxxx]

On 2022/4/19 上午2:14, Jacob Pan wrote:
> Hi zhangfei.gao@xxxxxxxxxxx,
>
> On Sat, 16 Apr 2022 09:43:07 +0800, "zhangfei.gao@xxxxxxxxxxx"
> <zhangfei.gao@xxxxxxxxxxx> wrote:
>
> > On 2022/4/16 上午5:00, Jacob Pan wrote:
> > > Hi zhangfei.gao@xxxxxxxxxxx,
> > >
> > > On Fri, 15 Apr 2022 19:52:03 +0800, "zhangfei.gao@xxxxxxxxxxx"
> > > <zhangfei.gao@xxxxxxxxxxx> 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