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:   Fri, 29 Apr 2022 15:53:57 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...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 2022/4/28 16:39, Jean-Philippe Brucker wrote:
> On Tue, Apr 26, 2022 at 04:31:57PM -0700, Dave Hansen wrote:
>> On 4/26/22 09:48, Jean-Philippe Brucker wrote:
>>> On Tue, Apr 26, 2022 at 08:27:00AM -0700, Dave Hansen wrote:
>>>> On 4/25/22 09:40, Jean-Philippe Brucker wrote:
>>>>> The problem is that we'd have to request the device driver to stop DMA
>>>>> before we can destroy the context and free the PASID. We did consider
>>>>> doing this in the release() MMU notifier, but there were concerns about
>>>>> blocking mmput() for too long (for example
>>>>> https://lore.kernel.org/linux-iommu/4d68da96-0ad5-b412-5987-2f7a6aa796c3@amd.com/
>>>>> though I think there was a more recent discussion). We also need to drain
>>>>> the PRI and fault queues to get rid of all references to that PASID.
>>>> Is the concern truly about blocking mmput() itself?  Or, is it about
>>>> releasing the resources associated with the mm?
>>> The latter I think, this one was about releasing pages as fast as possible
>>> if the process is picked by the OOM killer.
>>
>> We're tying the PASID to the life of the mm itself, not the mm's address
>> space.  That means the PASID should be tied to
>> mmgrab()/mmdrop()/mm->mm_count.
>>
>> 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?

Best regards,
baolu