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

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Date:   Sun, 18 Apr 2021 14:21:21 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     brouer@...hat.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, netdev@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        mcroce@...ux.microsoft.com, grygorii.strashko@...com,
        arnd@...nel.org, hch@....de, linux-snps-arc@...ts.infradead.org,
        mhocko@...nel.org, mgorman@...e.de
Subject: Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems

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@...aro.org>