Re: [PATCH] slub: Remove use of page->counter

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 11, 2018 at 11:26:06AM -0700, Matthew Wilcox wrote:
> I had a Thought.  And it seems to work:

Now I realised that slub doesn't use s_mem.  That gives us more space
to use for pages/pobjects.

struct page {
	unsigned long flags;
	union {		/* Five words */
		struct {	/* Page cache & anonymous pages */
			struct list_head lru;
			unsigned long private;
			struct address_space *mapping;
			pgoff_t index;
		};
		struct {	/* Slob */
			struct list_head slob_list;
			int units;
		};
		struct {	/* Slab */
			struct kmem_cache *slab_cache;
			void *freelist;
			void *s_mem;
			unsigned int active;
		};
		struct {	/* Slub */
			struct kmem_cache *slub_cache;
			/* Dword boundary */
			void *slub_freelist;
			unsigned short inuse;
			unsigned short objects:15;
			unsigned short frozen:1;
			struct page *next;
#ifdef CONFIG_64BIT
			int pobjects;
			int pages;
#endif
			short int pages;
			short int pobjects;
#endif
		};
		struct rcu_head rcu_head;
		... tail pages, page tables, etc, etc ...
	};
	union {
		atomic_t _mapcount;
		unsigned int page_type;
	};
	atomic_t _refcount;
	struct mem_cgroup *mem_cgroup;
};

> Now everybody gets 5 contiguous words to use as they want with the only
> caveat that they can't use bit 0 of the first word (PageTail).

^^^ still true ;-)

I'd want to change slob to use slob_list instead of ->lru.  Or maybe even do
something radical like _naming_ the struct in the union so we don't have to
manually namespace the names of the elements.