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

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Date:   Mon, 18 Apr 2022 11:14:56 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        jean-philippe <jean-philippe@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        iommu <iommu@...ts.linux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

Hi zhangfei.gao@...mail.com,

On Sat, 16 Apr 2022 09:43:07 +0800, "zhangfei.gao@...mail.com"
<zhangfei.gao@...mail.com> wrote:

> On 2022/4/16 上午5:00, Jacob Pan wrote:
> > Hi zhangfei.gao@...mail.com,
> >
> > On Fri, 15 Apr 2022 19:52:03 +0800, "zhangfei.gao@...mail.com"
> > <zhangfei.gao@...mail.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