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

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Message-ID: <79c397ee-b71b-470e-9184-401b4b96a0d2@arm.com>
Date:   Tue, 28 Nov 2023 22:59:33 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
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Subject: Re: [PATCH 06/16] iommu/dma: use page allocation function provided by
 iommu-pages.h

On 2023-11-28 10:50 pm, Pasha Tatashin wrote:
> On Tue, Nov 28, 2023 at 5:34 PM Robin Murphy <robin.murphy@....com> wrote:
>>
>> On 2023-11-28 8:49 pm, Pasha Tatashin wrote:
>>> Convert iommu/dma-iommu.c to use the new page allocation functions
>>> provided in iommu-pages.h.
>>
>> These have nothing to do with IOMMU pagetables, they are DMA buffers and
>> they belong to whoever called the corresponding dma_alloc_* function.
> 
> Hi Robin,
> 
> 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.

Thanks,
Robin.

> Since, I included DMA memory, I specifically removed mentioning of
> IOMMU page tables in the most of places, and only report it as IOMMU
> memory. However, since it is still bundled together with SecPageTables
> it can be confusing.
> 
> Pasha