On Thu, Jul 06, 2023, David Stevens wrote:

On Wed, Jul 5, 2023 at 7:17 PM Yu Zhang [off-list ref] wrote:

quoted

On Tue, Jul 04, 2023 at 04:50:50PM +0900, David Stevens wrote:

quoted

From: David Stevens <redacted>

Stop passing FOLL_GET to __kvm_follow_pfn. This allows the host to map
memory into the guest that is backed by un-refcounted struct pages - for
example, higher order non-compound pages allocated by the amdgpu driver
via ttm_pool_alloc_page.

I guess you mean the tail pages of the higher order non-compound pages?
And as to the head page, it is said to be set to one coincidentally[*],
and shall not be considered as refcounted.  IIUC, refcount of this head
page will be increased and decreased soon in hva_to_pfn_remapped(), so
this may not be a problem(?). But treating this head page differently,
as a refcounted one(e.g., to set the A/D flags), is weired.

Or maybe I missed some context, e.g., can the head page be allocted to
guest at all?

Yes, this is to allow mapping the tail pages of higher order
non-compound pages - I should have been more precise in my wording.
The head pages can already be mapped into the guest.

Recording for posterity (or to make an incorrect statment and get corrected),
because I recently had a conversation about the head page not actually being
refcounted.  (I can't remember with whom I had the conversation, but I'm pretty
sure it wasn't an imaginary friend).

Even though whatever allocates the page doesn't explicit refcount the head page,
__free_pages() will still do the right thing and (a) keep the head page around
until its last reference is put.  And my understanding is that even though it's
a "head" page, it's not a PG_head page, i.e. not a compound page and so is treated
as an order-0 page when KVM invoke put_page().

void __free_pages(struct page *page, unsigned int order)
{
	/* get PageHead before we drop reference */
	int head = PageHead(page);

	if (put_page_testzero(page))  <=== will evaluate false if KVM holds a ref
		free_the_page(page, order);
	else if (!head)  <=== will be false for non-compound pages
		while (order-- > 0)
			free_the_page(page + (1 << order), order);
}
EXPORT_SYMBOL(__free_pages);