[CRIU] [RFC PATCH v2 02/23] lazy-pages: track outstanding page faults

[Mike Rapoport]

Multithreaded applications may concurrently generate page faults at the
same address, which will cause creation of multiple requests for remote
page, and, consequently, confuse the page server on the dump side.
We can keep track on page fault requests in flight and ensure this way that
we request a page from the remote side only once.

Signed-off-by: Mike Rapoport <rppt at linux.vnet.ibm.com>
---
 criu/uffd.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/criu/uffd.c b/criu/uffd.c
index 557f112..050637c 100644
--- a/criu/uffd.c
+++ b/criu/uffd.c
@@ -55,10 +55,16 @@ struct lazy_iovec {
 
 struct lazy_pages_info;
 
+struct lp_req {
+	unsigned long addr;
+	struct list_head l;
+};
+
 struct lazy_pages_info {
 	int pid;
 
 	struct list_head iovs;
+	struct list_head reqs;
 
 	struct page_read pr;
 
@@ -85,6 +91,7 @@ static struct lazy_pages_info *lpi_init(void)
 
 	memset(lpi, 0, sizeof(*lpi));
 	INIT_LIST_HEAD(&lpi->iovs);
+	INIT_LIST_HEAD(&lpi->reqs);
 	INIT_LIST_HEAD(&lpi->l);
 	lpi->lpfd.revent = handle_uffd_event;
 
@@ -537,9 +544,19 @@ static int uffd_copy(struct lazy_pages_info *lpi, __u64 address, int nr_pages)
 
 static int complete_page_fault(struct lazy_pages_info *lpi, unsigned long vaddr, int nr)
 {
+	struct lp_req *req;
+
 	if (uffd_copy(lpi, vaddr, nr))
 		return -1;
 
+	list_for_each_entry(req, &lpi->reqs, l) {
+		if (req->addr == vaddr) {
+			list_del(&req->l);
+			xfree(req);
+			break;
+		}
+	}
+
 	return update_lazy_iovecs(lpi, vaddr, nr * PAGE_SIZE);
 }
 
@@ -635,6 +652,7 @@ static int handle_remaining_pages(struct lazy_pages_info *lpi)
 
 static int handle_page_fault(struct lazy_pages_info *lpi, struct uffd_msg *msg)
 {
+	struct lp_req *req;
 	__u64 address;
 	int ret;
 
@@ -642,6 +660,16 @@ static int handle_page_fault(struct lazy_pages_info *lpi, struct uffd_msg *msg)
 	address = msg->arg.pagefault.address & ~(page_size() - 1);
 	pr_debug("%d: #PF at 0x%llx\n", lpi->pid, address);
 
+	list_for_each_entry(req, &lpi->reqs, l)
+		if (req->addr == address)
+			return 0;
+
+	req = xzalloc(sizeof(*req));
+	if (!req)
+		return -1;
+	req->addr = address;
+	list_add(&req->l, &lpi->reqs);
+
 	ret = uffd_handle_pages(lpi, address, 1, PR_ASYNC | PR_ASAP);
 	if (ret < 0) {
 		pr_err("Error during regular page copy\n");
@@ -664,6 +692,9 @@ static int handle_uffd_event(struct epoll_rfd *lpfd)
 		return 1;
 
 	if (ret != sizeof(msg)) {
+		/* we've already handled the page fault for another thread */
+		if (errno == EAGAIN)
+			return 0;
 		if (ret < 0)
 			pr_perror("Can't read userfaultfd message");
 		else
-- 
1.9.1