Re: [PATCH 2/2] swiotlb: Enforce page alignment in swiotlb_alloc()

From: Christoph Hellwig <hch@lst.de>
To: Will Deacon <will@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>,
	linux-kernel@vger.kernel.org, kernel-team@android.com,
	iommu@lists.linux.dev,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Petr Tesarik <petr.tesarik1@huawei-partners.com>,
	Dexuan Cui <decui@microsoft.com>
Subject: Re: [PATCH 2/2] swiotlb: Enforce page alignment in swiotlb_alloc()
Date: Tue, 30 Jan 2024 15:34:37 +0100	[thread overview]
Message-ID: <20240130143437.GA31991@lst.de> (raw)
In-Reply-To: <20240129194939.GE12631@willie-the-truck>

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.