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 06:51:17 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        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

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