Re: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for allocations >= PAGE_SIZE

Christoph Hellwig
2024-02-29T13:33:51Z

On Thu, Feb 29, 2024 at 07:36:05AM +0000, Michael Kelley wrote:
> If there *is* a requirement for page alignment of page-size-or-greater
> requests, even when alloc_align_mask and min_align_mask are zero,
> we need to think about how to do that correctly, as that requirement
> is no longer met after Patch 1 of this series.

It has been historical behavior that all dma allocations are page
aligned (including in fact smaller than page sized ones that get
rounded up to a page size). The documentation actually (incorretly)
states an even stronger guarantee:

"The CPU virtual address and the DMA address are both
guaranteed to be aligned to the smallest PAGE_SIZE order which
is greater than or equal to the requested size. This invariant
exists (for example) to guarantee that if you allocate a chunk
which is smaller than or equal to 64 kilobytes, the extent of the
buffer you receive will not cross a 64K boundary."