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Date:   Fri, 15 Apr 2022 14:00:02 -0700
From:   Jacob Pan <jacob.jun.pan@linux.intel.com>
To:     "zhangfei.gao@foxmail.com" <zhangfei.gao@foxmail.com>
Cc:     Fenghua Yu <fenghua.yu@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        jean-philippe <jean-philippe@linaro.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        x86 <x86@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        iommu <iommu@lists.linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        jacob.jun.pan@linux.intel.com
Subject: 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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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?

Thanks,

Jacob