Re: [Chapter One] THP zones: the use cases of policy zones

[Matthew Wilcox <willy@infradead.org>]

On Thu, Feb 29, 2024 at 11:34:33AM -0700, Yu Zhao wrote:
> ZONE_MOVABLE becomes the first policy zone, followed by two new policy
> zones:
> 1. ZONE_NOSPLIT, which contains pages that are movable (inherited from
>    ZONE_MOVABLE) and restricted to a minimum order to be
>    anti-fragmentation. The latter means that they cannot be split down
>    below that order, while they are free or in use.
> 2. ZONE_NOMERGE, which contains pages that are movable and restricted
>    to an exact order. The latter means that not only is split
>    prohibited (inherited from ZONE_NOSPLIT) but also merge (see the
>    reason in Chapter Three), while they are free or in use.

These two zones end up solving a problem for memdescs.  So I'm in favour!
I added Option 5 to https://kernelnewbies.org/MatthewWilcox/BuddyAllocator

I think this patch needs to be split into more digestable chunks, but a
quick skim of it didn't reveal anything egregiously wrong.  I do still
have that question about the number of bits used for Zone in
page->flags.  Probably this all needs to be dependent on CONFIG_64BIT?