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

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Date:   Wed, 14 Apr 2021 15:52:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthew Wilcox' <willy@...radead.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Matteo Croce <mcroce@...ux.microsoft.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Arnd Bergmann <arnd@...nel.org>,
        "Christoph Hellwig" <hch@....de>
Subject: RE: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems

> Doing this fixes it:
> 
> +++ b/include/linux/types.h
> @@ -140,7 +140,7 @@ typedef u64 blkcnt_t;
>   * so they don't care about the size of the actual bus addresses.
>   */
>  #ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
> -typedef u64 dma_addr_t;
> +typedef u64 __attribute__((aligned(sizeof(void *)))) dma_addr_t;
>  #else
>  typedef u32 dma_addr_t;
>  #endif

I hate __packed so much I've been checking what it does!

If you add __packed to the dma_addr_t field inside the union
then gcc (at least) removes the pad from before it, but also
'remembers' the alignment that is enforced by other members
of the structure.

So you don't need the extra aligned(sizeof (void *)) since
that is implicit.

So in this case __packed probably has no side effects.
(Unless a 32bit arch has instructions for a 64bit read
that must not be on an 8n+4 boundary and the address is taken).

It also doesn't affect 64bit - since the previous field
forces 64bit alignment.

	David

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