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

[Vineet Gupta]

Hi Matthew,

On 4/16/21 7:45 PM, Matthew Wilcox 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.

FWIW, ARC doesn't require 8 byte alignment for 8 byte integers. This is 
only needed for 8-byte atomics due to the requirements of LLOCKD/SCOND 
instructions.

> 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>
> ---
>   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];
>   		};
>   		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;
> +	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;
>   }
>   
>   static inline bool is_page_pool_compiled_in(void)
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index ad8b0707af04..f014fd8c19a6 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -174,8 +174,10 @@ static void page_pool_dma_sync_for_device(struct page_pool *pool,
>   					  struct page *page,
>   					  unsigned int dma_sync_size)
>   {
> +	dma_addr_t dma_addr = page_pool_get_dma_addr(page);
> +
>   	dma_sync_size = min(dma_sync_size, pool->p.max_len);
> -	dma_sync_single_range_for_device(pool->p.dev, page->dma_addr,
> +	dma_sync_single_range_for_device(pool->p.dev, dma_addr,
>   					 pool->p.offset, dma_sync_size,
>   					 pool->p.dma_dir);
>   }
> @@ -226,7 +228,7 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
>   		put_page(page);
>   		return NULL;
>   	}
> -	page->dma_addr = dma;
> +	page_pool_set_dma_addr(page, dma);
>   
>   	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
>   		page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
> @@ -294,13 +296,13 @@ void page_pool_release_page(struct page_pool *pool, struct page *page)
>   		 */
>   		goto skip_dma_unmap;
>   
> -	dma = page->dma_addr;
> +	dma = page_pool_get_dma_addr(page);
>   
> -	/* When page is unmapped, it cannot be returned our pool */
> +	/* When page is unmapped, it cannot be returned to our pool */
>   	dma_unmap_page_attrs(pool->p.dev, dma,
>   			     PAGE_SIZE << pool->p.order, pool->p.dma_dir,
>   			     DMA_ATTR_SKIP_CPU_SYNC);
> -	page->dma_addr = 0;
> +	page_pool_set_dma_addr(page, 0);
>   skip_dma_unmap:
>   	/* This may be the last page returned, releasing the pool, so
>   	 * it is not safe to reference pool afterwards.