linux-kernel - Re: [PATCH 00/16] IOMMU memory observability

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Message-ID: <20231129002826.GG1312390@ziepe.ca>
Date:   Tue, 28 Nov 2023 20:28:26 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Pasha Tatashin <pasha.tatashin@...een.com>,
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Subject: Re: [PATCH 00/16] IOMMU memory observability

On Tue, Nov 28, 2023 at 04:25:03PM -0800, Yosry Ahmed wrote:

> > > Right, but as I mention above, if userspace starts depending on this
> > > equation, we won't be able to add any more classes of "secondary" page
> > > tables to SecPageTables. I'd like to avoid that if possible. We can do
> > > the subtraction in the kernel.
> >
> > What Sean had suggested was that SecPageTables was always intended to
> > account all the non-primary mmu memory used by page tables. If this is
> > the case we shouldn't be trying to break it apart into finer
> > counters. These are big picture counters, not detailed allocation by
> > owner counters.
> 
> Right, I agree with that, but if SecPageTables includes page tables
> from multiple sources, and it is observed to be suspiciously high, the
> logical next step is to try to find the culprit, right?

You can make that case already, if it is high wouldn't you want to
find the exact VMM process that was making it high?

It is a sign of fire, not a detailed debug tool.

Jason