linux-kernel - Re: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for allocations >= PAGE

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Date: Mon, 4 Mar 2024 15:32:03 +0000
From: Will Deacon <will@...nel.org>
To: Michael Kelley <mhklinux@...look.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kernel-team@...roid.com" <kernel-team@...roid.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	Dexuan Cui <decui@...rosoft.com>,
	Nicolin Chen <nicolinc@...dia.com>
Subject: Re: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for
 allocations >= PAGE_SIZE

On Thu, Feb 29, 2024 at 06:07:32AM +0000, Michael Kelley wrote:
> From: Will Deacon <will@...nel.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