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

[Matthew Wilcox]

On Sun, Apr 11, 2021 at 11:33:18AM +0100, Matthew Wilcox wrote:
> Basically, we have three aligned dwords here.  We can either alias with
> @flags and the first word of @lru, or the second word of @lru and @mapping,
> or @index and @private.  @flags is a non-starter.  If we use @mapping,
> then you have to set it to NULL before you free it, and I'm not sure
> how easy that will be for you.  If that's trivial, then we could use
> the layout:
> 
> 	unsigned long _pp_flags;
> 	unsigned long pp_magic;
> 	union {
> 		dma_addr_t dma_addr;    /* might be one or two words */
> 		unsigned long _pp_align[2];
> 	};
> 	unsigned long pp_pfmemalloc;
> 	unsigned long xmi;

I forgot about the munmap path.  That calls zap_page_range() which calls
set_page_dirty() which calls page_mapping().  If we use page->mapping,
that's going to get interpreted as an address_space pointer.

*sigh*.  Foiled at every turn.

I'm kind of inclined towards using two (or more) bits for PageSlab as
we discussed here:

https://lore.kernel.org/linux-mm/01000163efe179fe-d6270c58-eaba-482f-a6bd-334667250ef7-000000@email.amazonses.com/

so we have PageKAlloc that's true for PageSlab, PagePool, PageDMAPool,
PageVMalloc, PageFrag and maybe a few other kernel-internal allocations.

(see also here:)
https://lore.kernel.org/linux-mm/20180518194519.3820-18-willy@infradead.org/