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 15:34:36 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Baolu Lu <baolu.lu@...ux.intel.com>,
        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 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.

Thanks,
Jean

> 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
> 
>