Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

On Sat, Apr 17, 2021 at 09:22:40PM +0100, Matthew Wilcox wrote:
> On Sat, Apr 17, 2021 at 09:32:06PM +0300, Ilias Apalodimas wrote:
> > > +static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
> > > +{
> > > +	page->dma_addr[0] = addr;
> > > +	if (sizeof(dma_addr_t) > sizeof(unsigned long))
> > > +		page->dma_addr[1] = addr >> 16 >> 16;
> > 
> > The 'error' that was reported will never trigger right?
> > I assume this was compiled with dma_addr_t as 32bits (so it triggered the
> > compilation error), but the if check will never allow this codepath to run.
> > If so can we add a comment explaining this, since none of us will remember why
> > in 6 months from now?
> 
> That's right.  I compiled it all three ways -- 32-bit, 64-bit dma, 32-bit long
> and 64-bit.  The 32/64 bit case turn into:
> 
> 	if (0)
> 		page->dma_addr[1] = addr >> 16 >> 16;
> 
> which gets elided.  So the only case that has to work is 64-bit dma and
> 32-bit long.
> 
> I can replace this with upper_32_bits().
> 

Ok up to you, I don't mind either way and thanks for solving this!

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>