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

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Date: Wed, 21 Feb 2024 19:27:07 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Will Deacon <will@...nel.org>, akpm@...ux-foundation.org, alim.akhtar@...sung.com, 
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Subject: Re: [PATCH v3 10/10] iommu: account IOMMU allocated memory

> > > > > 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.
>
> Right, SeanC mentioned that the purpose of SecPageTables was to
> capture all non-mm page table radix allocations.
>
> > 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 think you should keep them together. I don't think we should be
> introducing new counters, in general.

Thanks Jason, I will keep it as-is. I will send a new version soon
with your comments addressed.

> Detailed memory profile should come from some kind of more dynamic and
> universal scheme. Hopefully that other giant thread about profiling
> will reach some conclusion.

+1! Memory profiling is going to be a very useful addition to the kernel.

Pasha