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

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Date: Mon, 29 Jan 2024 08:43:26 +0100
From: Petr Tesařík <petr@...arici.cz>
To: Christoph Hellwig <hch@....de>
Cc: Will Deacon <will@...nel.org>, 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, 29 Jan 2024 07:08:53 +0100
Christoph Hellwig <hch@....de> wrote:

> On Fri, Jan 26, 2024 at 03:19:56PM +0000, Will Deacon wrote:
> > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > index 25febb9e670c..92433ea9f2d2 100644
> > --- a/kernel/dma/swiotlb.c
> > +++ b/kernel/dma/swiotlb.c
> > @@ -1647,6 +1647,12 @@ struct page *swiotlb_alloc(struct device *dev, size_t size)
> >  		return NULL;
> >  
> >  	tlb_addr = slot_addr(pool->start, index);
> > +	if (!PAGE_ALIGNED(tlb_addr)) {
> > +		dev_WARN_ONCE(dev, 1, "Cannot return 'struct page *' for non page-aligned swiotlb addr 0x%pa.\n",
> > +			      &tlb_addr);
> > +		swiotlb_release_slots(dev, tlb_addr);
> > +		return NULL;
> > +	}
> >  
> >  	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.  That being said swiotlb obviously should never
> allocate unaligned addresses, but the placement of this check feels
> odd to me.  Also because it only catches swiotlb_alloc and not the
> map side.

We may have to rethink how alignment constraints are interpreted. See
also my reply to PATCH 1/2.

> Maybe just throw a WARN_ON_ONCE into slot_addr() ?

Yes.

Or, what if I write a KUnit test suite for swiotlb to combat this
constant stream of various regressions?

Petr T