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Date: Wed, 21 Feb 2024 13:29:19 +0000
From: Will Deacon <will@kernel.org>
To: Pasha Tatashin <pasha.tatashin@soleen.com>
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Subject: Re: [PATCH v3 10/10] iommu: account IOMMU allocated memory
Message-ID: <20240221132919.GC7273@willie-the-truck>
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On Fri, Feb 16, 2024 at 02:48:00PM -0500, Pasha Tatashin wrote:
> On Fri, Feb 16, 2024 at 12:58 PM Will Deacon <will@kernel.org> wrote:
> >
> > On Tue, Feb 13, 2024 at 10:44:53AM -0500, Pasha Tatashin wrote:
> > > > >  SecPageTables
> > > > > -              Memory consumed by secondary page tables, this currently
> > > > > -              currently includes KVM mmu allocations on x86 and arm64.
> > > > > +              Memory consumed by secondary page tables, this currently includes
> > > > > +              KVM mmu and IOMMU allocations on x86 and arm64.
> > >
> > > Hi Will,
> > >
> > > > While I can see the value in this for IOMMU mappings managed by VFIO,
> > > > doesn't this end up conflating that with the normal case of DMA domains?
> > > > For systems that e.g. rely on an IOMMU for functional host DMA, it seems
> > > > wrong to subject that to accounting constraints.
> > >
> > > The accounting constraints are only applicable when GFP_KERNEL_ACCOUNT
> > > is passed to the iommu mapping functions. We do that from the vfio,
> > > iommufd, and vhost. Without this flag, the memory useage is reported
> > > in /proc/meminfo as part of  SecPageTables field, but not constrained
> > > in cgroup.
> >
> > Thanks, Pasha, that explanation makes sense. I still find it bizarre to
> > include IOMMU allocations from the DMA API in SecPageTables though, and
> > I worry that it will confuse people who are using that metric as a way
> > to get a feeling for how much memory is being used by KVM's secondary
> > page-tables. As an extreme example, having a non-zero SecPageTables count
> > without KVM even compiled in is pretty bizarre.
> 
> I agree; I also prefer a new field in /proc/meminfo named
> 'IOMMUPageTables'. This is what I proposed at LPC, but I was asked to
> reuse the existing 'SecPageTables' field instead. The rationale was
> that 'secondary' implies not only KVM page tables, but any other
> non-regular page tables.
> 
> I would appreciate the opinion of IOMMU maintainers on this: is it
> preferable to bundle the information with 'SecPageTables' or maintain
> a separate field?

I personally find it confusing to add all IOMMU page-table allocations
to SecPageTables, considering that userspace could be using that today
with a reasonable expectation that it's concerned only with virtual
machine overhead. However, if the opposite conclusion was reached at LPC,
then I really don't want to re-open the discussion and derail your
patchset.

Will