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, 30 Apr 2022 15:33:17 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Peter Zijlstra <peterz@...radead.org>, robin.murphy@....com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        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>, zhangfei.gao@...aro.org,
        Thomas Gleixner <tglx@...utronix.de>, will@...nel.org
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

On 2022/4/30 06:19, Fenghua Yu wrote:
> Hi, Jean and Baolu,
> 
> On Fri, Apr 29, 2022 at 03:34:36PM +0100, Jean-Philippe Brucker wrote:
>> On Fri, Apr 29, 2022 at 06:51:17AM -0700, Fenghua Yu wrote:
>>> 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().
>>
>> Yes, although for Arm I realized the mmu_notifier grab wasn't sufficient
>> so I sent a separate fix that should go in 5.18 as well
>> https://lore.kernel.org/linux-iommu/20220426130444.300556-1-jean-philippe@linaro.org/
>> The Arm driver still touches the arch mm context after mmu_notifier_put().
>> I don't think X86 has that problem.
> 
> I think so too. On X86, the mm is not used after mmu_notifier_unregister().
> 
> In case of supervisor mode SVM (i.e. svm is bound to init_mm), the code
> is right too because init_mm and its PASID cannot be dropped and
> mmu_notifier_register()/mmu_notifier_unregister() are not called.
> 
> So I think no extra fix patch is needed on X86.

Thanks, Fenghua and Jean. It's clear to me now.

Jean, another quick question about the iommu_sva_bind_device()

/**
  * iommu_sva_bind_device() - Bind a process address space to a device
  * @dev: the device
  * @mm: the mm to bind, caller must hold a reference to it
  * @drvdata: opaque data pointer to pass to bind callback

This interface requires the caller to take a reference to mm. Which
reference should it take, mm->mm_count or mm->mm_users? It's better to
make it explicit in this comment.

Best regards,
baolu