Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems

[Matthew Wilcox <willy@infradead.org>]

On Sat, Apr 17, 2021 at 12:31:37PM +0200, Arnd Bergmann wrote:
> On Fri, Apr 16, 2021 at 5:27 PM Matthew Wilcox <willy@infradead.org> wrote:
> > diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> > index b5b195305346..db7c7020746a 100644
> > --- a/include/net/page_pool.h
> > +++ b/include/net/page_pool.h
> > @@ -198,7 +198,17 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
> >
> >  static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
> >  {
> > -       return page->dma_addr;
> > +       dma_addr_t ret = page->dma_addr[0];
> > +       if (sizeof(dma_addr_t) > sizeof(unsigned long))
> > +               ret |= (dma_addr_t)page->dma_addr[1] << 32;
> > +       return ret;
> > +}
> 
> Have you considered using a PFN type address here? I suspect you
> can prove that shifting the DMA address by PAGE_BITS would
> make it fit into an 'unsigned long' on all 32-bit architectures with
> 64-bit dma_addr_t. This requires that page->dma_addr to be
> page aligned, as well as fit into 44 bits. I recently went through the
> maximum address space per architecture to define a
> MAX_POSSIBLE_PHYSMEM_BITS, and none of them have more than
> 40 here, presumably the same is true for dma address space.

I wouldn't like to make that assumption.  I've come across IOMMUs (maybe
on parisc?  powerpc?) that like to encode fun information in the top
few bits.  So we could get it down to 52 bits, but I don't think we can
get all the way down to 32 bits.  Also, we need to keep the bottom bit
clear for PageTail, so that further constrains us.

Anyway, I like the "two unsigned longs" approach I posted yesterday,
but thanks for the suggestion.