Re: Struct page proposal

From: Vlastimil Babka
Date: Mon Sep 27 2021 - 13:48:47 EST

On 9/23/21 03:21, Kent Overstreet wrote:
> So if we have this:
> struct page {
> unsigned long allocator;
> unsigned long allocatee;
> };
> The allocator field would be used for either a pointer to slab/slub's state, if
> it's a slab page, or if it's a buddy allocator page it'd encode the order of the
> allocation - like compound order today, and probably whether or not the
> (compound group of) pages is free.

The "free page in buddy allocator" case will be interesting to implement.
What the buddy allocator uses today is:

- PageBuddy - determine if page is free; a page_type (part of mapcount
field) today, could be a bit in "allocator" field that would have to be 0 in
all other "page is allocated" contexts.
- nid/zid - to prevent merging accross node/zone boundaries, now part of
page flags
- buddy order
- a list_head (reusing the "lru") to hold the struct page on the appropriate
free list, which has to be double-linked so page can be taken from the
middle of the list instantly

Won't be easy to cram all that into two unsigned long's, or even a single
one. We should avoid storing anything in the free page itself. Allocating
some external structures to track free pages is going to have funny
bootstrap problems. Probably a major redesign would be needed...