Re: [PATCH v6 6/6] swiotlb: Reinstate page-alignment for mappings >= PAGE_SIZE

[Robin Murphy]

On 2024-03-08 4:38 pm, Petr Tesařík wrote:
> On Fri, 8 Mar 2024 16:08:01 +0000
> Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> > On 2024-03-08 3:28 pm, Will Deacon wrote:
> > > For swiotlb allocations >= PAGE_SIZE, the slab search historically
> > > adjusted the stride to avoid checking unaligned slots. This had the
> > > side-effect of aligning large mapping requests to PAGE_SIZE, but that
> > > was broken by 0eee5ae10256 ("swiotlb: fix slot alignment checks").
> > >
> > > Since this alignment could be relied upon drivers, reinstate PAGE_SIZE
> > > alignment for swiotlb mappings >= PAGE_SIZE.
> >
> > This seems clear enough to keep me happy now, thanks! And apologies that
> > I managed to confuse even myself in the previous thread...
> >
> > Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>
>
> I thought we agreed that this stricter alignment is unnecessary:
>
>    https://lore.kernel.org/linux-iommu/20240305140833.GC3659@xxxxxx/

No, that was about dma_alloc_coherent() again (and TBH I'm not sure we 
should actually relax it anyway, since there definitely are callers who 
rely on size-alignment beyond PAGE_SIZE, however they're typically going 
to be using the common implementations which end up in alloc_pages() or 
CMA and so do offer that, rather than the oddball ones which don't - 
e.g. we're never going to be allocating SMMUv3 Stream Tables out of some 
restricted pool via the emergency swiotlb_alloc() path). If anywhere, 
the place to argue that point would be patch #3 (which as mentioned I'd 
managed to forget about before...)

This one's just about preserving a SWIOTLB-specific behaviour which has 
the practical effect of making SWIOTLB a bit less visible to dma_map_*() 
callers. The impact of keeping this is fairly low, so seems preferable 
to the risk of facing issues 2 or 3 years down the line when someone 
finally upgrades their distro and their data gets eaten because it turns 
out some obscure driver should really have been updated to use 
min_align_mask.

Thanks,
Robin.

> But if everybody else wants to have it...
>
> Petr T
>
> > > Reported-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> > > Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> > > ---
> > >    kernel/dma/swiotlb.c | 18 +++++++++++-------
> > >    1 file changed, 11 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > > index c381a7ed718f..c5851034523f 100644
> > > --- a/kernel/dma/swiotlb.c
> > > +++ b/kernel/dma/swiotlb.c
> > > @@ -992,6 +992,17 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
> > >    	BUG_ON(!nslots);
> > >    	BUG_ON(area_index >= pool->nareas);
> > >    
> > > +	/*
> > > +	 * Historically, swiotlb allocations >= PAGE_SIZE were guaranteed to be
> > > +	 * page-aligned in the absence of any other alignment requirements.
> > > +	 * 'alloc_align_mask' was later introduced to specify the alignment
> > > +	 * explicitly, however this is passed as zero for streaming mappings
> > > +	 * and so we preserve the old behaviour there in case any drivers are
> > > +	 * relying on it.
> > > +	 */
> > > +	if (!alloc_align_mask && !iotlb_align_mask && alloc_size >= PAGE_SIZE)
> > > +		alloc_align_mask = PAGE_SIZE - 1;
> > > +
> > >    	/*
> > >    	 * Ensure that the allocation is at least slot-aligned and update
> > >    	 * 'iotlb_align_mask' to ignore bits that will be preserved when
> > > @@ -1006,13 +1017,6 @@ static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool
> > >    	 */
> > >    	stride = get_max_slots(max(alloc_align_mask, iotlb_align_mask));
> > >    
> > > -	/*
> > > -	 * For allocations of PAGE_SIZE or larger only look for page aligned
> > > -	 * allocations.
> > > -	 */
> > > -	if (alloc_size >= PAGE_SIZE)
> > > -		stride = umax(stride, PAGE_SHIFT - IO_TLB_SHIFT + 1);
> > > -
> > >    	spin_lock_irqsave(&area->lock, flags);
> > >    	if (unlikely(nslots > pool->area_nslabs - area->used))
> > >    		goto not_found;