page-read: Support reading by chunks

This is needed for the case when we get data from image-cache/proxy socket.
Signed-off-by: Rodrigo Bruno <rbruno@gsd.inesc-id.pt>
Signed-off-by: Katerina Koukiou <k.koukiou@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/pagemap.c

@@ -249,6 +249,7 @@ static int read_local_page(struct page_read *pr, unsigned long vaddr,
 {
 	int fd = img_raw_fd(pr->pi);
 	int ret;
+	size_t curr = 0;
 
 	/*
 	 * Flush any pending async requests if any not to break the
@@ -258,11 +259,16 @@ static int read_local_page(struct page_read *pr, unsigned long vaddr,
 		return -1;
 
 	pr_debug("\tpr%u Read page from self %lx/%"PRIx64"\n", pr->id, pr->cvaddr, pr->pi_off);
-	ret = pread(fd, buf, len, pr->pi_off);
-	if (ret != len) {
+	while (1) {
+		ret = pread(fd, buf + curr, len - curr, pr->pi_off + curr);
+		if (ret < 1) {
 			pr_perror("Can't read mapping page %d", ret);
 			return -1;
 		}
+		curr += ret;
+		if (curr == len)
+			break;
+	}
 
 	if (opts.auto_dedup) {
 		ret = punch_hole(pr, pr->pi_off, len, false);