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Date:   Wed, 14 Apr 2021 22:35:56 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Jesper Dangaard Brouer <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 <ilias.apalodimas@linaro.org>,
        Matteo Croce <mcroce@linux.microsoft.com>,
        Grygorii Strashko <grygorii.strashko@ti.com>,
        Arnd Bergmann <arnd@kernel.org>, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 1/1] mm: Fix struct page layout on 32-bit systems
Message-ID: <20210414213556.GY2531743@casper.infradead.org>
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On Wed, Apr 14, 2021 at 09:13:22PM +0200, Jesper Dangaard Brouer wrote:
> (If others want to reproduce).  First I could not reproduce on ARM32.
> Then I found out that enabling CONFIG_XEN on ARCH=arm was needed to
> cause the issue by enabling CONFIG_ARCH_DMA_ADDR_T_64BIT.

hmmm ... you should be able to provoke it by enabling ARM_LPAE,
which selects PHYS_ADDR_T_64BIT, and

config ARCH_DMA_ADDR_T_64BIT
        def_bool 64BIT || PHYS_ADDR_T_64BIT

>  struct page {
>         long unsigned int          flags;                /*     0     4 */
> 
>         /* XXX 4 bytes hole, try to pack */
> 
>         union {
>                 struct {
>                         struct list_head lru;            /*     8     8 */
>                         struct address_space * mapping;  /*    16     4 */
>                         long unsigned int index;         /*    20     4 */
>                         long unsigned int private;       /*    24     4 */
>                 };                                       /*     8    20 */
>                 struct {
>                         dma_addr_t dma_addr;             /*     8     8 */
>                 };                                       /*     8     8 */
[...]
>         } __attribute__((__aligned__(8)));               /*     8    24 */
>         union {
>                 atomic_t           _mapcount;            /*    32     4 */
>                 unsigned int       page_type;            /*    32     4 */
>                 unsigned int       active;               /*    32     4 */
>                 int                units;                /*    32     4 */
>         };                                               /*    32     4 */
>         atomic_t                   _refcount;            /*    36     4 */
> 
>         /* size: 40, cachelines: 1, members: 4 */
>         /* sum members: 36, holes: 1, sum holes: 4 */
>         /* forced alignments: 1, forced holes: 1, sum forced holes: 4 */
>         /* last cacheline: 40 bytes */
> } __attribute__((__aligned__(8)));

If you also enable CONFIG_MEMCG or enough options to make
LAST_CPUPID_NOT_IN_PAGE_FLAGS true, you'll end up with another 4-byte
hole at the end.