Hi Matthew,

On Sat, Apr 17, 2021 at 03:45:22AM +0100, Matthew Wilcox wrote:
> 
> Replacement patch to fix compiler warning.
> 
> From: "Matthew Wilcox (Oracle)" <wi...@infradead.org>
> Date: Fri, 16 Apr 2021 16:34:55 -0400
> Subject: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems
> To: bro...@redhat.com
> Cc: linux-ker...@vger.kernel.org,
>     linux...@kvack.org,
>     net...@vger.kernel.org,
>     linuxppc-dev@lists.ozlabs.org,
>     linux-arm-ker...@lists.infradead.org,
>     linux-m...@vger.kernel.org,
>     ilias.apalodi...@linaro.org,
>     mcr...@linux.microsoft.com,
>     grygorii.stras...@ti.com,
>     a...@kernel.org,
>     h...@lst.de,
>     linux-snps-...@lists.infradead.org,
>     mho...@kernel.org,
>     mgor...@suse.de
> 
> 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) <wi...@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];
>               };
>               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;


The 'error' that was reported will never trigger right?
I assume this was compiled with dma_addr_t as 32bits (so it triggered the
compilation error), but the if check will never allow this codepath to run.
If so can we add a comment explaining this, since none of us will remember why
in 6 months from now?

>  }
>  
>  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.
> -- 
> 2.30.2
> 

Thanks
/Ilias

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