[RFC v1 6/9] io_uring: introduce reference tracking for user pages.

[Jonathan Lemon <jonathan.lemon@xxxxxxxxx>]

This is currently a WIP.

If only part of a page is used by a skb fragment, and then provided
to the user, the page should not be reused by the kernel until all
sub-page fragments are not in use.

If only full pages are used (and not sub-page fragments), then this
code shouldn't be needed.

Signed-off-by: Jonathan Lemon <jonathan.lemon@xxxxxxxxx>
---
 io_uring/zctap.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/io_uring/zctap.c b/io_uring/zctap.c
index 58b4c5417650..9db3421fb9fa 100644
--- a/io_uring/zctap.c
+++ b/io_uring/zctap.c
@@ -183,9 +183,36 @@ struct ifq_region {
 	int			count;
 	int			imu_idx;
 	int			nr_pages;
+	u8			*page_uref;
 	struct page		*page[];
 };
 
+static void io_add_page_uref(struct ifq_region *ifr, u16 pgid)
+{
+	if (WARN_ON(!ifr))
+		return;
+
+	if (WARN_ON(pgid < ifr->imu_idx))
+		return;
+
+	ifr->page_uref[pgid - ifr->imu_idx]++;
+}
+
+static bool io_put_page_last_uref(struct ifq_region *ifr, u64 addr)
+{
+	int idx;
+
+	if (WARN_ON(addr < ifr->start || addr > ifr->end))
+		return false;
+
+	idx = (addr - ifr->start) >> PAGE_SHIFT;
+
+	if (WARN_ON(!ifr->page_uref[idx]))
+		return false;
+
+	return --ifr->page_uref[idx] == 0;
+}
+
 int io_provide_ifq_region_prep(struct io_kiocb *req,
 			       const struct io_uring_sqe *sqe)
 {
@@ -244,6 +271,11 @@ int io_provide_ifq_region(struct io_kiocb *req, unsigned int issue_flags)
 	if (!ifr)
 		return -ENOMEM;
 
+	ifr->page_uref = kvmalloc_array(nr_pages, sizeof(u8), GFP_KERNEL);
+	if (!ifr->page_uref) {
+		kvfree(ifr);
+		return -ENOMEM;
+	}
 
 	ifr->nr_pages = nr_pages;
 	ifr->imu_idx = idx;
@@ -261,6 +293,7 @@ int io_provide_ifq_region(struct io_kiocb *req, unsigned int issue_flags)
 		info = zctap_page_info(r->bgid, idx + i, id);
 		set_page_private(page, info);
 		ifr->page[i] = page;
+		ifr->page_uref[i] = 0;
 	}
 
 	WRITE_ONCE(r->ifq->region,  ifr);
@@ -273,6 +306,7 @@ int io_provide_ifq_region(struct io_kiocb *req, unsigned int issue_flags)
 		set_page_private(page, 0);
 	}
 
+	kvfree(ifr->page_uref);
 	kvfree(ifr);
 
 	return -EEXIST;
-- 
2.30.2