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

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On 17/04/2021 01.07, Matthew Wilcox (Oracle) wrote:
> 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>
> ---
>  include/linux/mm_types.h |  4 ++--
>  include/net/page_pool.h  | 12 +++++++++++-
>  net/core/page_pool.c     | 12 +++++++-----
>  3 files changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 6613b26a8894..5aacc1c10a45 100644
> --- 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];

Shouldn't that member get another name (_dma_addr?) to be sure the
buildbots catch every possible (ab)user and get them turned into the new
accessors? Sure, page->dma_addr is now "pointer to unsigned long"
instead of "dma_addr_t", but who knows if there's a
"(long)page->dma_addr" somewhere?

Rasmus