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:   Sat, 16 Apr 2022 09:43:07 +0800
From:   "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.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>
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit



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;

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

       // only daemon process

Thanks

>
> Thanks,
>
> Jacob