Re: [PATCH 0/2] Fix double allocation in swiotlb_alloc()

[Will Deacon <will@kernel.org>]

On Fri, Jan 26, 2024 at 05:20:59PM +0100, Petr Tesařík wrote:
> On Fri, 26 Jan 2024 15:19:54 +0000
> Will Deacon <will@kernel.org> wrote:
> 
> > Hi folks,
> > 
> > These two patches fix a nasty double allocation problem in swiotlb_alloc()
> > and add a diagnostic to help catch any similar issues in future. This was
> > a royal pain to track down and I've had to make a bit of a leap at the
> > correct alignment semantics (i.e. iotlb_align_mask vs alloc_align_mask).
> 
> Welcome to the club. I believe you had to re-discover what I described here:
> 
>   https://lore.kernel.org/linux-iommu/20231108101347.77cab795@meshulam.tesarici.cz/

Lucky me...

> The relevant part would be this:
> 
>   To sum it up, there are two types of alignment:
> 
>   1. specified by a device's min_align_mask; this says how many low
>      bits of a buffer's physical address must be preserved,
> 
>   2. specified by allocation size and/or the alignment parameter;
>      this says how many low bits in the first IO TLB slot's physical
>      address must be zero.  
> 
> Fix for that has been sitting on my TODO list for too long. :-(

FWIW, it did _used_ to work (or appear to work), so it would be good to
at least get it back to the old behaviour if nothing else.

Anyway, cheers for reviewing the patches. I'll go through your comments
now...

Will