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/ (local)

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/ (local)

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