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:   Thu, 28 Apr 2022 09:39:27 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>, will@...nel.org,
        robin.murphy@....com, zhangfei.gao@...aro.org
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

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.

Thanks,
Jean