Re: [PATCH] mm: fix struct page layout on 32-bit systems

[Matthew Wilcox <willy@infradead.org>]

On Sun, May 16, 2021 at 09:29:41AM -0700, Linus Torvalds wrote:
> On Sun, May 16, 2021 at 5:19 AM Matthew Wilcox (Oracle)
> <willy@infradead.org> 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'?

Nobody's been willing to guarantee that all 32-bit architectures keep the
top 20 bits clear for their DMA addresses.  I've certainly seen hardware
(maybe PA-RISC?  MIPS?) which uses the top few bits of the DMA address to
indicate things like "coherent" or "bypasses IOMMU".  Rather than trying
to find out, I thought this was the safer option.  It only affects
32-bit architectures with PAE, and I'd rather not introduce a shift on
64-bit architectures to work around a 32-bit PAE problem.