page-read: Properly handle overlapping pagemaps in stack

If the pagemap we read at some point covers two (or more) pagemaps
in parent images, we cannot just call the parent page read for all
those pages. Instead, we must call parent page read with shorter
requests so that it handles its own pagemaps properly.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

page-read.c

@@ -91,7 +91,7 @@ static void skip_pagemap_pages(struct page_read *pr, unsigned long len)
 	if (!len)
 		return;
 
-	pr_debug("\tpr%u Skip %lx bytes from page-dump\n", pr->id, len);
+	pr_debug("\tpr%u Skip %lu bytes from page-dump\n", pr->id, len);
 	if (!pr->pe->in_parent)
 		lseek(img_raw_fd(pr->pi), len, SEEK_CUR);
 	pr->cvaddr += len;
@@ -148,22 +148,56 @@ static int read_pagemap_page(struct page_read *pr, unsigned long vaddr, int nr,
 	int ret;
 	unsigned long len = nr * PAGE_SIZE;
 
+	pr_info("pr%u Read %lx %u pages\n", pr->id, vaddr, nr);
 	pagemap_bound_check(pr->pe, vaddr, nr);
 
 	if (pr->pe->in_parent) {
-		pr_debug("\tpr%u Read page %lx from parent\n", pr->id, vaddr);
-		ret = seek_pagemap_page(pr->parent, vaddr, true);
-		if (ret <= 0)
-			return -1;
-		ret = read_pagemap_page(pr->parent, vaddr, nr, buf);
-		if (ret == -1)
-			return ret;
+		struct page_read *ppr = pr->parent;
+
+		/*
+		 * Parent pagemap at this point entry may be shorter
+		 * than the current vaddr:nr needs, so we have to
+		 * carefully 'split' the vaddr:nr into pieces and go
+		 * to parent page-read with the longest requests it
+		 * can handle.
+		 */
+
+		do {
+			int p_nr;
+
+			pr_debug("\tpr%u Read from parent\n", pr->id);
+			ret = seek_pagemap_page(ppr, vaddr, true);
+			if (ret <= 0)
+				return -1;
+
+			/*
+			 * This is how many pages we have in the parent
+			 * page_read starting from vaddr. Go ahead and
+			 * read as much as we can.
+			 */
+			p_nr = ppr->pe->nr_pages - (vaddr - ppr->pe->vaddr) / PAGE_SIZE;
+			pr_info("\tparent has %u pages in\n", p_nr);
+			if (p_nr > nr)
+				p_nr = nr;
+
+			ret = read_pagemap_page(ppr, vaddr, p_nr, buf);
+			if (ret == -1)
+				return ret;
+
+			/*
+			 * OK, let's see how much data we have left and go
+			 * to parent page-read again for the next pagemap
+			 * entry.
+			 */
+			nr -= p_nr;
+			vaddr += p_nr * PAGE_SIZE;
+			buf += p_nr * PAGE_SIZE;
+		} while (nr);
 	} else {
 		int fd = img_raw_fd(pr->pi);
 		off_t current_vaddr = lseek(fd, 0, SEEK_CUR);
 
-		pr_debug("\tpr%u Read page %lx from self %lx/%"PRIx64"\n", pr->id,
-				vaddr, pr->cvaddr, current_vaddr);
+		pr_debug("\tpr%u Read page from self %lx/%"PRIx64"\n", pr->id, pr->cvaddr, current_vaddr);
 		ret = read(fd, buf, len);
 		if (ret != len) {
 			pr_perror("Can't read mapping page %d", ret);