linux-kernel - Re: [PATCH 06/16] iommu/dma: use page allocation function provided by iommu-pages.h

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Date:   Tue, 28 Nov 2023 18:08:11 -0500
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Robin Murphy <robin.murphy@....com>
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Subject: Re: [PATCH 06/16] iommu/dma: use page allocation function provided by iommu-pages.h

> > This is true, however, we want to account and observe the pages
> > allocated by IOMMU subsystem for DMA buffers, as they are essentially
> > unmovable locked pages. Should we separate IOMMU memory from KVM
> > memory all together and add another field to /proc/meminfo, something
> > like "iommu -> iommu pagetable and dma memory", or do we want to
> > export DMA memory separately from IOMMU page tables?
>
> These are not allocated by "the IOMMU subsystem", they are allocated by
> the DMA API. Even if you want to claim that a driver pinning memory via
> iommu_dma_ops is somehow different from the same driver pinning the same
> amount of memory via dma-direct when iommu.passthrough=1, it's still
> nonsense because you're failing to account the pages which iommu_dma_ops
> gets from CMA, dma_common_alloc_pages(), dynamic SWIOTLB, the various
> pools, and so on.

I see, IOMMU variants are used only for discontiguous allocations, and
the common ones are defined outside of driver/iommu. Alright, I can
remove all the changes for all no-page table related IOMMU
allocations.

Pasha