Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 29 Apr 2022 06:51:17 -0700
From:   Fenghua Yu <fenghua.yu@intel.com>
To:     Baolu Lu <baolu.lu@linux.intel.com>
Cc:     Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Peter Zijlstra <peterz@infradead.org>, robin.murphy@arm.com,
        Dave Hansen <dave.hansen@linux.intel.com>,
        x86 <x86@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        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>, zhangfei.gao@linaro.org,
        Thomas Gleixner <tglx@linutronix.de>, will@kernel.org
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit
Message-ID: <YmvtVRlwVJjStXc0@fyu1.sc.intel.com>
References: <Yman5hLomw9/c+bi@myrica>
 <76ec6342-0d7c-7c7b-c132-2892e4048fa1@intel.com>
 <YmavoKkVu+hd+x0M@myrica>
 <22b659c7-e972-7a56-2bd7-8df3b4820d4e@intel.com>
 <YmbO/l5IwfBCHrl8@myrica>
 <8c044e49-74bb-df56-8a60-663013c0910e@intel.com>
 <YmgiQZZyFxsJ+9um@myrica>
 <63582490-a794-fd11-0380-44b27cc660b7@intel.com>
 <YmpSv48aPIbVXqGi@myrica>
 <044595e6-e5d0-26c2-af8e-fc9d06906179@linux.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <044595e6-e5d0-26c2-af8e-fc9d06906179@linux.intel.com>
Precedence: bulk

Hi, Baolu,

On Fri, Apr 29, 2022 at 03:53:57PM +0800, Baolu Lu wrote:
> On 2022/4/28 16:39, Jean-Philippe Brucker wrote:
> > > The address space is what the OOM killer is after.  That gets refcounted
> > > with mmget()/mmput()/mm->mm_users.  The OOM killer is satiated by the
> > > page freeing done in __mmput()->exit_mmap().
> > > 
> > > Also, all the VMAs should be gone after exit_mmap().  So, even if
> > > vma->vm_file was holding a reference to a device driver, that reference
> > > should be gone by the time __mmdrop() is actually freeing the PASID.
> > 
> > I agree with all that. The concern was about tearing down the PASID in the
> > IOMMU and device from the release() MMU notifier, which would happen in
> > exit_mmap(). But doing the teardown at or before __mmdrop() is fine. And
> > since the IOMMU drivers need to hold mm->mm_count anyway between bind()
> > and unbind(), I think Fenghua's fix works.
> 
> But I didn't find mmgrab()/mmdrop() get called in both arm and intel
> IOMMU drivers.
> 
> $ git grep mmgrab drivers/iommu/
> [no output]
> 
> Do we need to add these in a separated fix patch, or I missed anything
> here?

On both ARM and X86, sva_bind() calls mmu_notifier_register()->mmgrab() and
sva_unbind() calls mmu_notifier_unregister()/mmu_notifier_put()->mmdrop().

So mm->mm_count are already counted in existing ARM and X86 binding and
unbinding. The fix patch just frees the PASID in __mmdrop() after
no more mm->mm_count.

There is no need to add extra  mmgrab() and mmdrop() pair.

Thanks.

-Fenghua