linux-kernel - Re: [PATCH v3 10/10] iommu: account IOMMU allocated memory

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Date: Fri, 16 Feb 2024 14:48:00 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Will Deacon <will@...nel.org>
Cc: akpm@...ux-foundation.org, alim.akhtar@...sung.com, alyssa@...enzweig.io, 
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Subject: Re: [PATCH v3 10/10] iommu: account IOMMU allocated memory

On Fri, Feb 16, 2024 at 12:58 PM Will Deacon <will@...nel.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?

Pasha