With the upcoming shrink of struct page to 4 words, we need a plan for
handling PageMovable. Ideally this does not involve memory allocation,
and is a relatively simple change from what we have now. To shrink
struct page beyond 4 words, we'll need a better plan, but I think this
will do for the next few months.
The current proposed layout for struct page is:
struct page {
unsigned long flags;
union {
struct list_head buddy_list;
struct list_head pcp_list;
struct {
unsigned long memdesc;
union {
unsigned long private;
atomic_t _mapcount;
};
};
};
int _refcount;
};
My proposal for movable non-folio pages is:
* memdesc is used to point to struct movable_operations (these will
need to be aligned to 16 bytes, but I think that's fine)
* private is used to point to the next page in the list
* These pages are refcounted
* We retain a "lock" bit in page->flags
The most disruptive part of this is that we can't use a list_head any
more. I don't think a singly-linked list is prohibitive, but either
we need to switch folios to also use a singly-linked list, or we need
to handle folios & movable pages separately. I think the former is
probably best.
