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

    Replacement patch to fix compiler warning.

    From: "Matthew Wilcox (Oracle)" <willy@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: brouer@redhat.com
    Cc: linux-kernel@vger.kernel.org,
    linux-mm@kvack.org,
    netdev@vger.kernel.org,
    linuxppc-dev@lists.ozlabs.org,
    linux-arm-kernel@lists.infradead.org,
    linux-mips@vger.kernel.org,
    ilias.apalodimas@linaro.org,
    mcroce@linux.microsoft.com,
    grygorii.strashko@ti.com,
    arnd@kernel.org,
    hch@lst.de,
    linux-snps-arc@lists.infradead.org,
    mhocko@kernel.org,
    mgorman@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) <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];
    };
    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.
    --
    2.30.2
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