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

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Message-Id: <15A1A033-73DC-4ACA-87B0-3535E9236F7F@gmail.com>
Date:   Tue, 8 Nov 2022 12:37:28 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/4] mm: introduce 'encoded' page pointers with embedded
 extra bits

On Nov 8, 2022, at 11:41 AM, Linus Torvalds <torvalds@...ux-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 ?