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

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Message-ID: <bf27a35f-e568-f8e3-6aa9-45b129db0b09@redhat.com>
Date:   Wed, 16 Nov 2022 10:15:49 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Alexander Gordeev <agordeev@...ux.ibm.com>
Subject: Re: [PATCH 1/4] mm: introduce 'encoded' page pointers with embedded
 extra bits

On 09.11.22 21:30, Linus Torvalds 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.
> 
> Note that this makes the alignment of 'struct page' explicit even for
> the case where CONFIG_HAVE_ALIGNED_STRUCT_PAGE is not set.  That is
> entirely redundant in almost all cases, since the page structure already
> contains several word-sized entries.
> 
> However, on m68k, the alignment of even 32-bit data is just 16 bits, and
> as such in theory the alignment of 'struct page' could be too.  So let's
> just make it very very explicit that the alignment needs to be at least
> 32 bits, giving us a guarantee of two unused low bits in the pointer.
> 
> Now, in practice, our page struct array is aligned much more than that
> anyway, even on m68k, and our existing code in mm/mlock.c obviously
> already depended on that.  But since the whole point of this change is
> to be careful about the type system when hiding extra bits in the
> pointer, let's also be explicit about the assumptions we make.
> 
> NOTE! This is being very careful in another way too: it has a build-time
> assertion that the 'flags' added to the page pointer actually fit in the
> two bits.  That means that this helper must be inlined, and can only be
> used in contexts where the compiler can statically determine that the
> value fits in the available bits.
> 
> Link: https://lore.kernel.org/all/Y2tKixpO4RO6DgW5@tuxmaker.boeblingen.de.ibm.com/
> Cc: Alexander Gordeev <agordeev@...ux.ibm.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb