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

[Matthew Wilcox <willy@infradead.org>]

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.