linux-kernel - Re: [PATCH 2/2] swiotlb: Enforce page alignment in swiotlb

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Date: Tue, 30 Jan 2024 15:34:37 +0100
From: Christoph Hellwig <hch@....de>
To: Will Deacon <will@...nel.org>
Cc: Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
	kernel-team@...roid.com, iommu@...ts.linux.dev,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	Dexuan Cui <decui@...rosoft.com>
Subject: Re: [PATCH 2/2] swiotlb: Enforce page alignment in swiotlb_alloc()

On Mon, Jan 29, 2024 at 07:49:40PM +0000, Will Deacon wrote:
> > >  	return pfn_to_page(PFN_DOWN(tlb_addr));
> > 
> > So PFN_DOWN aligns the address and thus per se converting the unaligned
> > address isn't a problem.
> 
> Hmm, I'm not sure I follow why it isn't a problem. If the first 2KiB slot
> of the 4KiB page has already been allocated to somebody else, isn't it a
> big problem to align down like that? Maybe I should word the warning
> message a bit better -- how about:

But the problem is that it's used, not that we can't create a page
from a non-aligned address.

> 
>   "Cannot allocate pages from non page-aligned swiotlb addr 0x%pa.\n"

That sounds better.