Re: [PATCH 1/4] mm: introduce 'encoded' page pointers with embedded extra bits

Nadav Amit
2022-11-08T20:37:35Z

On Nov 8, 2022, at 11:41 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote:

> We already have this notion in parts of the MM code (see the mlock code
> with the LRU_PAGE and NEW_PAGE) bits, but I'm going to introduce a new
> case, and I refuse to do the same thing we've done before where we just
> put bits in the raw pointer and say it's still a normal pointer.
>
> So this introduces a 'struct encoded_page' pointer that cannot be used
> for anything else than to encode a real page pointer and a couple of
> extra bits in the low bits. That way the compiler can trivially track
> the state of the pointer and you just explicitly encode and decode the
> extra bits.

I tested again all of the patches with the PoC. They pass.

>
> +struct encoded_page;
> +#define ENCODE_PAGE_BITS 3ul
> +static inline struct encoded_page *encode_page(struct page *page, unsigned long flags)
> +{
> + return (struct encoded_page *)(flags | (unsigned long)page);
> +}
> +
> +static inline bool encoded_page_flags(struct encoded_page *page)
> +{
> + return ENCODE_PAGE_BITS & (unsigned long)page;
> +}

I think this one wants to be some unsigned, as otherwise why have
ENCODE_PAGE_BITS as 3ul ?