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

[David Laight <David.Laight@ACULAB.COM>]

From: Matthew Wilcox
> Sent: 14 April 2021 22:36
> 
> 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

Adding __packed here will remove the 4 byte hole before the union
and the compiler seems clever enough to know that anything following
a 'long' must also be 'long' aligned.
So you don't get anything horrid like byte accesses.
On 64bit dma_addr will remain 64bit aligned.
On arm32 dma_addr will be 32bit aligned - but forcing two 32bit access
won't make any difference.

So definitely the only simple fix.

	David

> >                                            ;             /*     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.

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