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SubjectRe: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems
On Wed, Apr 14, 2021 at 12:50:52PM +0100, Matthew Wilcox wrote:
> On Wed, Apr 14, 2021 at 10:10:44AM +0200, Jesper Dangaard Brouer wrote:
> > Yes, indeed! - And very frustrating. It's keeping me up at night.
> > I'm dreaming about 32 vs 64 bit data structures. My fitbit stats tell
> > me that I don't sleep well with these kind of dreams ;-)
>
> Then you're going to love this ... even with the latest patch, there's
> still a problem. Because dma_addr_t is still 64-bit aligned _as a type_,
> that forces the union to be 64-bit aligned (as we already knew and worked
> around), but what I'd forgotten is that forces the entirety of struct
> page to be 64-bit aligned. Which means ...
>
> /* size: 40, cachelines: 1, members: 4 */
> /* padding: 4 */
> /* forced alignments: 1 */
> /* last cacheline: 40 bytes */
> } __attribute__((__aligned__(8)));
>
> .. that we still have a hole! It's just moved from being at offset 4
> to being at offset 36.
>
> > That said, I think we need to have a quicker fix for the immediate
> > issue with 64-bit bit dma_addr on 32-bit arch and the misalignment hole
> > it leaves[3] in struct page. In[3] you mention ppc32, does it only
> > happens on certain 32-bit archs?
>
> AFAICT it happens on mips32, ppc32, arm32 and arc. It doesn't happen
> on x86-32 because dma_addr_t is 32-bit aligned.
>
> Doing this fixes it:
>
> +++ b/include/linux/types.h
> @@ -140,7 +140,7 @@ typedef u64 blkcnt_t;
> * so they don't care about the size of the actual bus addresses.
> */
> #ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
> -typedef u64 dma_addr_t;
> +typedef u64 __attribute__((aligned(sizeof(void *)))) dma_addr_t;
> #else
> typedef u32 dma_addr_t;
> #endif
>
> > I'm seriously considering removing page_pool's support for doing/keeping
> > DMA-mappings on 32-bit arch's. AFAIK only a single driver use this.
>
> ... if you're going to do that, then we don't need to do this.

FWIW I already proposed that to Matthew in private a few days ago...
II am not even sure the AM572x has that support. I'd much prefer getting rid
of it as well, instead of overcomplicating the struct for a device noone is
going to need.

Cheers
/Ilias

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