[kbuild-all] Re: Bogus struct page layout on 32-bit

On Sat, Apr 10, 2021 at 4:44 AM Matthew Wilcox <willy(a)infradead.org&gt; wrote: > + dma_addr_t dma_addr __packed; > }; > struct { /* slab, slob and slub */ > union { > > but I don't know if GCC is smart enough to realise that dma_addr is now > on an 8 byte boundary and it can use a normal instruction to access it, > or whether it'll do something daft like use byte loads to access it. > > We could also do: > > + dma_addr_t dma_addr __packed __aligned(sizeof(void *)); > > and I see pahole, at least sees this correctly: > > struct { > long unsigned int _page_pool_pad; /* 4 4 */ > dma_addr_t dma_addr __attribute__((__aligned__(4))); /* 8 8 */ > } __attribute__((__packed__)) __attribute__((__aligned__(4))); > > This presumably affects any 32-bit architecture with a 64-bit phys_addr_t > / dma_addr_t. Advice, please? I've tried out what gcc would make of this: https://godbolt.org/z/aTEbxxbG3 struct page { short a; struct { short b; long long c __attribute__((packed, aligned(2))); } __attribute__((packed)); } __attribute__((aligned(8))); In this structure, 'c' is clearly aligned to eight bytes, and gcc does realize that it is safe to use the 'ldrd' instruction for 32-bit arm, which is forbidden on struct members with less than 4 byte alignment. However, it also complains that passing a pointer to 'c' into a function that expects a 'long long' is not allowed because alignof(c) is only '2' here. (I used 'short' here because I having a 64-bit member misaligned by four bytes wouldn't make a difference to the instructions on Arm, or any other 32-bit architecture I can think of, regardless of the ABI requirements).