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

[David Laight <David.Laight@ACULAB.COM>]

From: Geert Uytterhoeven
> Sent: 20 April 2021 08:40
> 
> Hi Willy,
> 
> On Sat, Apr 17, 2021 at 4:49 AM Matthew Wilcox <willy@infradead.org> wrote:
> > Replacement patch to fix compiler warning.
> >
> > 32-bit architectures which expect 8-byte alignment for 8-byte integers
> > and need 64-bit DMA addresses (arc, 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.
> >
> > Fix this by storing the dma_addr_t in one or two adjacent unsigned longs.
> > This restores the alignment to that of an unsigned long, and also fixes a
> > potential problem where (on a big endian platform), the bit used to denote
> > PageTail could inadvertently get set, and a racing get_user_pages_fast()
> > could dereference a bogus compound_head().
> >
> > Fixes: c25fff7171be ("mm: add dma_addr_t to struct page")
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> 
> Thanks for your patch!
> 
> > --- a/include/linux/mm_types.h
> > +++ 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?
> 
> Would
> 
>     unsigned long _dma_addr[sizeof(dma_addr_t) / sizeof(unsigned long)];
> 
> work?
> 
> Or will the compiler become too overzealous, and warn about the use
> of ...[1] below, even when unreachable?
> I wouldn't mind an #ifdef instead of an if () in the code below, though.

You could use [ARRAY_SIZE()-1] instead of [1].
Or, since IIRC it is the last member of that specific struct, define as:
		unsigned long dma_addr[];

...
> > -       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] << 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;

Or:
	ret |= page->dma_addr[1] + 0ull << 32;

Which relies in integer promotion rather than a cast.

	David

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