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

[Geert Uytterhoeven]

Hi Willy,

On Sat, Apr 17, 2021 at 4:49 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> Replacement patch to fix compiler warning.
>
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Date: Fri, 16 Apr 2021 16:34:55 -0400
> Subject: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems
> To: brouer@xxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx,
>     linux-mm@xxxxxxxxx,
>     netdev@xxxxxxxxxxxxxxx,
>     linuxppc-dev@xxxxxxxxxxxxxxxx,
>     linux-arm-kernel@xxxxxxxxxxxxxxxxxxx,
>     linux-mips@xxxxxxxxxxxxxxx,
>     ilias.apalodimas@xxxxxxxxxx,
>     mcroce@xxxxxxxxxxxxxxxxxxx,
>     grygorii.strashko@xxxxxx,
>     arnd@xxxxxxxxxx,
>     hch@xxxxxx,
>     linux-snps-arc@xxxxxxxxxxxxxxxxxxx,
>     mhocko@xxxxxxxxxx,
>     mgorman@xxxxxxx
>
> 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.

>                 };
>                 struct {        /* slab, slob and slub */
>                         union {
> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> index b5b195305346..ad6154dc206c 100644
> --- a/include/net/page_pool.h
> +++ b/include/net/page_pool.h
> @@ -198,7 +198,17 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
>
>  static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
>  {
> -       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;

> +       return ret;
> +}
> +
> +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;

... but we do have upper_32_bits() for a 32-bit right shift.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds