linux-kernel - Re: [PATCH 08/16] iommu/fsl: use page allocation function provided by iommu-pages.h

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Date:   Tue, 28 Nov 2023 19:50:37 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     Robin Murphy <robin.murphy@....com>, akpm@...ux-foundation.org,
        alex.williamson@...hat.com, alim.akhtar@...sung.com,
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        iommu@...ts.linux.dev, jasowang@...hat.com,
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Subject: Re: [PATCH 08/16] iommu/fsl: use page allocation function provided
 by iommu-pages.h

On Tue, Nov 28, 2023 at 06:00:13PM -0500, Pasha Tatashin wrote:
> On Tue, Nov 28, 2023 at 5:53 PM Robin Murphy <robin.murphy@....com> wrote:
> >
> > On 2023-11-28 8:49 pm, Pasha Tatashin wrote:
> > > Convert iommu/fsl_pamu.c to use the new page allocation functions
> > > provided in iommu-pages.h.
> >
> > Again, this is not a pagetable. This thing doesn't even *have* pagetables.
> >
> > Similar to patches #1 and #2 where you're lumping in configuration
> > tables which belong to the IOMMU driver itself, as opposed to pagetables
> > which effectively belong to an IOMMU domain's user. But then there are
> > still drivers where you're *not* accounting similar configuration
> > structures, so I really struggle to see how this metric is useful when
> > it's so completely inconsistent in what it's counting :/
> 
> The whole IOMMU subsystem allocates a significant amount of kernel
> locked memory that we want to at least observe. The new field in
> vmstat does just that: it reports ALL buddy allocator memory that
> IOMMU allocates. However, for accounting purposes, I agree, we need to
> do better, and separate at least iommu pagetables from the rest.
> 
> We can separate the metric into two:
> iommu pagetable only
> iommu everything
> 
> or into three:
> iommu pagetable only
> iommu dma
> iommu everything
> 
> What do you think?

I think I said this at LPC - if you want to have fine grained
accounting of memory by owner you need to go talk to the cgroup people
and come up with something generic. Adding ever open coded finer
category breakdowns just for iommu doesn't make alot of sense.

You can make some argument that the pagetable memory should be counted
because kvm counts it's shadow memory, but I wouldn't go into further
detail than that with hand coded counters..

Jason