linux-kernel - Re: Struct page proposal

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Date:   Mon, 27 Sep 2021 19:05:26 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: Struct page proposal

On Mon, Sep 27, 2021 at 07:48:15PM +0200, Vlastimil Babka wrote:
> 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...

Wait, why do we want to avoid using the memory that we're allocating?