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

Will Deacon
2024-03-04T15:32:08Z

On Thu, Feb 29, 2024 at 06:07:32AM +0000, Michael Kelley wrote:
> From: Will Deacon <will@kernel.org> Sent: Wednesday, February 28, 2024 5:40 AM
> >
> > For swiotlb allocations >= PAGE_SIZE, the slab search historically
> > adjusted the stride to avoid checking unaligned slots. However, this is
> > no longer needed now that the code around it has evolved and the
> > stride is calculated from the required alignment.
> >
> > Either 'alloc_align_mask' is used to specify the allocation alignment or
> > the DMA 'min_align_mask' is used to align the allocation with 'orig_addr'.
> > At least one of these masks is always non-zero.
>
> I think the patch is correct, but this justification is not. alloc_align_mask
> and the DMA min_align_mask are often both zero. While the NVMe
> PCI driver sets min_align_mask, SCSI disk drivers do not (except for the
> Hyper-V synthetic SCSI driver). When both are zero, presumably
> there are no alignment requirements, so a stride of 1 is appropriate.

Sorry, yes, I messed up the commit message here as I was trying to reason
through the allocation case separately from the mapping case. However, I
need to digest the rest of this thread before doing the obvious fix...

Will