On 27.09.21 20:12, Matthew Wilcox wrote:
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?
Immediately comes to mind:
* Free page reporting via virtio-balloon
* CMA on s390x (see arch_free_page())
* Free page poisoning
* init_on_free
--
Thanks,
David / dhildenb
