lazy-pages: interleave #PF handling with transfers of remaining pages

Currently we poll userfaultfd for page faults and if there were no page
faults during 5 seconds we stop monitoring the userfaultfd and start
copying remaining pages chunk by chunk.
If a page fault occurs during the copy, the faulting process will be stuck
until the page it accessed would be copied to its address space.
This patch limits the initial "page fault only" stage to 1 second instead
of 5, and interleaves non-blocking poll of userfaultfd with copying of the
remaining memory afterwards.

travis-ci: success for lazy-pages: interleave #PF handling with transfers of remaining pages
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/uffd.c

@@ -70,7 +70,6 @@ struct lazy_pages_info {
 	struct list_head l;
 
 	void *buf;
-	bool remaining;
 };
 
 static LIST_HEAD(lpis);
@@ -547,9 +546,6 @@ static int complete_page_fault(struct lazy_pages_info *lpi, unsigned long vaddr,
 	if (uffd_copy(lpi, vaddr, nr))
 		return -1;
 
-	if (lpi->remaining)
-		return 0;
-
 	return update_lazy_iovecs(lpi, vaddr, nr * PAGE_SIZE);
 }
 
@@ -631,18 +627,16 @@ static int handle_remaining_pages(struct lazy_pages_info *lpi)
 	struct lazy_iovec *lazy_iov;
 	int nr_pages, err;
 
-	lpi->remaining = true;
-
-	lpi->pr.reset(&lpi->pr);
+	if (list_empty(&lpi->iovs))
+		return 0;
 
-	list_for_each_entry(lazy_iov, &lpi->iovs, l) {
-		nr_pages = lazy_iov->len / PAGE_SIZE;
+	lazy_iov = list_first_entry(&lpi->iovs, struct lazy_iovec, l);
+	nr_pages = lazy_iov->len / PAGE_SIZE;
 
-		err = uffd_handle_pages(lpi, lazy_iov->base, nr_pages, 0);
-		if (err < 0) {
-			pr_err("Error during UFFD copy\n");
-			return -1;
-		}
+	err = uffd_handle_pages(lpi, lazy_iov->base, nr_pages, 0);
+	if (err < 0) {
+		pr_err("Error during UFFD copy\n");
+		return -1;
 	}
 
 	return 0;
@@ -717,27 +711,37 @@ static int lazy_pages_summary(struct lazy_pages_info *lpi)
 	return 0;
 }
 
-#define POLL_TIMEOUT 5000
+#define POLL_TIMEOUT 1000
 
 static int handle_requests(int epollfd, struct epoll_event *events, int nr_fds)
 {
 	struct lazy_pages_info *lpi;
+	int poll_timeout = POLL_TIMEOUT;
 	int ret;
 
-	ret = epoll_run_rfds(epollfd, events, nr_fds, POLL_TIMEOUT);
-	if (ret < 0)
-		goto out;
-
+	for (;;) {
+		bool remaining = false;
 
-	pr_debug("switching from request to copy mode\n");
-	pr_debug("Handle remaining pages\n");
-	list_for_each_entry(lpi, &lpis, l) {
-		ret = handle_remaining_pages(lpi);
-		if (ret < 0) {
-			pr_err("Error during remaining page copy\n");
-			ret = 1;
+		ret = epoll_run_rfds(epollfd, events, nr_fds, poll_timeout);
+		if (ret < 0)
 			goto out;
+
+		if (poll_timeout)
+			pr_debug("Start handling remaining pages\n");
+
+		poll_timeout = 0;
+		list_for_each_entry(lpi, &lpis, l) {
+			if (lpi->copied_pages < lpi->total_pages) {
+				remaining = true;
+				ret = handle_remaining_pages(lpi);
+				if (ret < 0)
+					goto out;
+				break;
+			}
 		}
+
+		if (!remaining)
+			break;
 	}
 
 	list_for_each_entry(lpi, &lpis, l)