The ultimate goal of the folios project is to turn struct page into:

struct page {
        unsigned long memdesc;
};

The memdesc contains a 4 bit type field that describes what the remaining 60/28 bits are used for.

Type 0

Type 0 is used for allocations which will never be freed. The next four bits distinguish what kind of allocation this is:

The high bits are used to store zone/node/... information, as is done today with the page flags. Bits 8-13 are also used to store the order of the allocation.

Memdesc pointers

All structs pointed to from a memdesc must be allocated from a slab which has its alignment set to 16 bytes (in order to allow the bottom 4 bits to be used for the type). That implies that they are a multiple of 16 bytes in size. The slab must also have the TYPESAFE_BY_RCU flag set as some page walkers will attempt to look up the memdesc from the page while holding only the RCU read lock.

Other than struct buddy, all memdescs must have a pointer to struct buddy as the first word. See below.

struct buddy

struct buddy {
        unsigned long flags;
        struct list_head buddy_list;
        struct page *first;
};

This is used by the buddy allocator to track free pages. It is also used when the user does not need to store significant auxiliary information. The flags word contains node/zone/section/etc information as page->flags does today. However the lower bits are quite different:

Allocating memory

Device drivers that do not touch the contents of struct page can continue calling alloc_pages() as they do today. They will get back a struct page pointer which will have a struct buddy memdesc, but they won't care.

We'll add a new buddy_alloc() which will return the buddy memdesc pointer instead. Each allocator can change the subtype from 'Free' to whatever subtype it has.

We'll add a new memdesc_alloc_pages() family which allocate the memory and set each page->memdesc to the passed-in memdesc. So each memdesc allocator will first use slab to allocate a memdesc, then allocate the pages that point to that memdesc. As an optimisation, if sizeof(struct my_memdesc) is <= sizeof(struct buddy), we can avoid allocating a new memdesc and cast the pointer returned from buddy_alloc().

There's a minor recursion problem for the slab memdesc. This can be avoided by special-casing the struct slab allocation; any time we need to allocate a new slab for the slab memdesc cache, we _do not_ allocate a struct slab for it; we use the first object in the allocated memory for its own struct slab.

I don't know how we'll handle alloc_pages_exact().

Freeing memory

Folios (file/anon/ksm) have a refcount. These should be freed with folio_put(). Other memdescs may not have a refcount,

Allocations which use a buddy memdesc are simply passed back to the buddy allocator. Other memdescs will copy the buddy pointer from the memdesc before RCU freeing it, and update each struct page with the buddy memdesc.

Memory control group

Mapping memory into userspace

File, anon memory and KSM memory is rmappable. The rmap does not apply to other kinds of memory (networking, device driver, vmalloc, etc). These kinds of memory should be added to VM_MIXEDMAP or VM_PFNMAP mappings only.