On Sun, May 16, 2021 at 5:19 AM Matthew Wilcox (Oracle)
<willy@xxxxxxxxxxxxx> wrote:
>
> 32-bit architectures which expect 8-byte alignment for 8-byte integers and
> need 64-bit DMA addresses (arm, mips, ppc) had their struct page
> inadvertently expanded in 2019. When the dma_addr_t was added, it forced
> the alignment of the union to 8 bytes, which inserted a 4 byte gap between
> 'flags' and the union.
So I already have this in my tree, but this stable submission made me go "Hmm".
Why do we actually want a full 64-bit DMA address on 32-bit architectures here?
It strikes me that the address is page-aligned, and I suspect we could
just use a 32-bit "DMA page frame number" instead in 'struct page'?
So instead of that odd
+ if (sizeof(dma_addr_t) > sizeof(unsigned long))
+ ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;
maybe we could just do effectively
ret + (dma_addr_t)page->dma_frame_nr << PAGE_SHIFT;
and simplify this all. We could do it on 64-bit too, just to not have
any opdd special cases (even if we'd have the full 64 bits available).
Hmm?
Linus
