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

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Message-ID: <20210420113216.GA3596236@casper.infradead.org>
Date:   Tue, 20 Apr 2021 12:32:16 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, netdev <netdev@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        mcroce@...ux.microsoft.com,
        Grygorii Strashko <grygorii.strashko@...com>,
        Arnd Bergmann <arnd@...nel.org>,
        Christoph Hellwig <hch@....de>,
        arcml <linux-snps-arc@...ts.infradead.org>,
        Michal Hocko <mhocko@...nel.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems

On Tue, Apr 20, 2021 at 09:39:54AM +0200, Geert Uytterhoeven wrote:
> > +++ b/include/linux/mm_types.h
> > @@ -97,10 +97,10 @@ struct page {
> >                 };
> >                 struct {        /* page_pool used by netstack */
> >                         /**
> > -                        * @dma_addr: might require a 64-bit value even on
> > +                        * @dma_addr: might require a 64-bit value on
> >                          * 32-bit architectures.
> >                          */
> > -                       dma_addr_t dma_addr;
> > +                       unsigned long dma_addr[2];
> 
> So we get two 64-bit words on 64-bit platforms, while only one is
> needed?

Not really.  This is part of the 5-word union in struct page, so the space
ends up being reserved anyway, event if it's not "assigned" to 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] << 16 << 16;
> 
> We don't seem to have a handy macro for a 32-bit left shift yet...
> 
> But you can also avoid the warning using
> 
>     ret |= (u64)page->dma_addr[1] << 32;

Sure.  It doesn't really matter which way we eliminate the warning;
the code is unreachable.

> > +{
> > +       page->dma_addr[0] = addr;
> > +       if (sizeof(dma_addr_t) > sizeof(unsigned long))
> > +               page->dma_addr[1] = addr >> 16 >> 16;
> 
> ... but we do have upper_32_bits() for a 32-bit right shift.

Yep, that's what my current tree looks like.