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

[David Hildenbrand]

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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> Cc: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb