Re: [RFC PATCH v2 12/26] KVM: arm64: Introduce a Hyp buddy page allocator

On Wed, Feb 03, 2021 at 06:33:30PM +0000, Quentin Perret wrote:

On Tuesday 02 Feb 2021 at 18:13:08 (+0000), Will Deacon wrote:

quoted

On Fri, Jan 08, 2021 at 12:15:10PM +0000, Quentin Perret wrote:

quoted

+ *   __find_buddy(pool, page 0, order 0) => page 1
+ *   __find_buddy(pool, page 0, order 1) => page 2
+ *   __find_buddy(pool, page 1, order 0) => page 0
+ *   __find_buddy(pool, page 2, order 0) => page 3
+ */
+static struct hyp_page *__find_buddy(struct hyp_pool *pool, struct hyp_page *p,
+				     unsigned int order)
+{
+	phys_addr_t addr = hyp_page_to_phys(p);
+
+	addr ^= (PAGE_SIZE << order);
+	if (addr < pool->range_start || addr >= pool->range_end)
+		return NULL;

Are these range checks only needed because the pool isn't required to be
an exact power-of-2 pages in size? If so, maybe it would be more
straightforward to limit the max order on a per-pool basis depending upon
its size?

More importantly, it is because pages outside of the pool are not
guaranteed to be covered by the hyp_vmemmap, so I really need to make
sure I don't dereference them.

Wouldn't having a per-pool max order help with that?

quoted

quoted

+	return hyp_phys_to_page(addr);
+}
+
+static void __hyp_attach_page(struct hyp_pool *pool,
+			      struct hyp_page *p)
+{
+	unsigned int order = p->order;
+	struct hyp_page *buddy;
+
+	p->order = HYP_NO_ORDER;

Why is this needed?

If p->order is say 3, I may be able to coalesce with the buddy of order
3 to form a higher order page of order 4. And that higher order page
will be represented by the 'first' of the two order-3 pages (let's call
it the head), and the other order 3 page (let's say the tail) will be
assigned 'HYP_NO_ORDER'.

And basically at this point I don't know if 'p' is going be the head or
the tail, so I set it to HYP_NO_ORDER a priori so I don't have to think
about this in the loop below. Is that helping?

I suppose this could use more comments as well ...

Comments would definitely help, but perhaps even having a simple function to
do the coalescing, which you could call from the loop body and which would
deal with marking the tail pages as HYP_NO_ORDER?

quoted

quoted

+	for (; order < HYP_MAX_ORDER; order++) {
+		/* Nothing to do if the buddy isn't in a free-list */
+		buddy = __find_buddy(pool, p, order);
+		if (!buddy || list_empty(&buddy->node) || buddy->order != order)

Could we move the "buddy->order" check into __find_buddy()?

I think might break __hyp_extract_page() below. The way I think about
__find_buddy() is as a low level function which gives you the buddy page
blindly if it exists in the hyp_vmemmap, and it's up to the callers to
decide whether the buddy is in the right state for their use or not.

Just feels a bit backwards having __find_buddy() take an order parameter,
yet then return a page of the wrong order! __hyp_extract_page() always
passes the p->order as the order, so I think it would be worth having a
separate function that just takes the pool and the page for that.

Will