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Date:   Mon, 27 Sep 2021 19:48:15 +0200
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To:     Kent Overstreet <kent.overstreet@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        David Howells <dhowells@redhat.com>
References: <YUvWm6G16+ib+Wnb@moria.home.lan>
From:   Vlastimil Babka <vbabka@suse.cz>
Subject: Re: Struct page proposal
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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...