Re: [PATCH v9 08/96] mm: Fix struct page layout on 32-bit systems

[Vlastimil Babka]

On 5/5/21 5:05 PM, Matthew Wilcox (Oracle) wrote:
> 32-bit architectures which expect 8-byte alignment for 8-byte integers
> and need 64-bit DMA addresses (arm, mips, ppc) had their struct page
> inadvertently expanded in 2019.  When the dma_addr_t was added, it forced
> the alignment of the union to 8 bytes, which inserted a 4 byte gap between
> 'flags' and the union.
> 
> Fix this by storing the dma_addr_t in one or two adjacent unsigned longs.
> This restores the alignment to that of an unsigned long.  We always
> store the low bits in the first word to prevent the PageTail bit from
> being inadvertently set on a big endian platform.  If that happened,
> get_user_pages_fast() racing against a page which was freed and
> reallocated to the page_pool could dereference a bogus compound_head(),
> which would be hard to trace back to this cause.
> 
> Fixes: c25fff7171be ("mm: add dma_addr_t to struct page")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Acked-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
> Acked-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>