Re: Struct page proposal

[Matthew Wilcox <willy@infradead.org>]

On Mon, Sep 27, 2021 at 02:09:49PM -0400, Kent Overstreet wrote:
> On Mon, Sep 27, 2021 at 07:05:26PM +0100, Matthew Wilcox wrote:
> > 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?
> 
> The issue is where to stick the state for free pages. If that doesn't fit in two
> ulongs, then we'd need a separate allocation, which means slab needs to be up
> and running before free pages are initialized.

But the thing we're allocating is at least PAGE_SIZE bytes in size.
Why is "We should avoid storing anything in the free page itself" true?