From: Geert Uytterhoeven
> Sent: 20 April 2021 08:40
>
> Hi Willy,
>
> On Sat, Apr 17, 2021 at 4:49 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>
> 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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