restore: Restore memory from stack of snapshots

This one is a little bit more tricky that dump. On restore we
should open the chain of pagemap-s (by "parent" links). Then for
every pagemap with in_parent set we should go to parent and ask
for the page. Parent, in turn, should properly determine where
the respective page is in his pagemap and position the page.img's
position respectively.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/page-read.h

@@ -10,6 +10,11 @@ struct page_read {
 
 	int fd;
 	int fd_pg;
+
+	PagemapEntry *pe;
+	struct page_read *parent;
+	unsigned long cvaddr;
+	unsigned id; /* for logging */
 };
 
 int open_page_read(int pid, struct page_read *);

page-read.c

@@ -47,34 +47,132 @@ static int get_pagemap(struct page_read *pr, struct iovec *iov)
 
 	iov->iov_base = decode_pointer(pe->vaddr);
 	iov->iov_len = pe->nr_pages * PAGE_SIZE;
-	pagemap_entry__free_unpacked(pe, NULL);
+
+	pr->pe = pe;
+	pr->cvaddr = (unsigned long)iov->iov_base;
+
+	if (pe->in_parent && !pr->parent) {
+		pr_err("No parent for snapshot pagemap\n");
+		return -1;
+	}
 
 	return 1;
 }
 
 static void put_pagemap(struct page_read *pr)
 {
+	pagemap_entry__free_unpacked(pr->pe, NULL);
+}
+
+static int read_pagemap_page(struct page_read *pr, unsigned long vaddr, void *buf);
+
+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);
+	if (!pr->pe->in_parent)
+		lseek(pr->fd_pg, len, SEEK_CUR);
+	pr->cvaddr += len;
+}
+
+static int read_pagemap_page_from_parent(struct page_read *pr, unsigned long vaddr, void *buf)
+{
+	int ret;
+	struct iovec iov;
+
+	if (pr->pe) {
+		iov.iov_base = decode_pointer(pr->pe->vaddr);
+		iov.iov_len = pr->pe->nr_pages * PAGE_SIZE;
+	} else
+		goto new_pagemap;
+
+	while (1) {
+		unsigned long iov_end;
+
+		BUG_ON(vaddr < pr->cvaddr);
+		iov_end = (unsigned long)iov.iov_base + iov.iov_len;
+
+		if (iov_end <= vaddr) {
+			skip_pagemap_pages(pr, iov_end - pr->cvaddr);
+			put_pagemap(pr);
+new_pagemap:
+			ret = get_pagemap(pr, &iov);
+			if (ret <= 0)
+				return -1;
+
+			continue;
+		}
+
+		skip_pagemap_pages(pr, vaddr - pr->cvaddr);
+		return read_pagemap_page(pr, vaddr, buf);
+	}
 }
 
 static int read_pagemap_page(struct page_read *pr, unsigned long vaddr, void *buf)
 {
 	int ret;
 
-	ret = read(pr->fd_pg, buf, PAGE_SIZE);
-	if (ret != PAGE_SIZE) {
-		pr_err("Can'r read mapping page %d\n", ret);
-		return -1;
+	if (pr->pe->in_parent) {
+		pr_debug("\tpr%u Read page %lx from parent\n", pr->id, vaddr);
+		ret = read_pagemap_page_from_parent(pr->parent, vaddr, buf);
+	} else {
+		pr_debug("\tpr%u Read page %lx from self %lx/%lx\n", pr->id,
+				vaddr, pr->cvaddr, lseek(pr->fd_pg, 0, SEEK_CUR));
+		ret = read(pr->fd_pg, buf, PAGE_SIZE);
+		if (ret != PAGE_SIZE) {
+			pr_err("Can'r read mapping page %d\n", ret);
+			return -1;
+		}
 	}
 
+	pr->cvaddr += PAGE_SIZE;
+
 	return 1;
 }
 
 static void close_page_read(struct page_read *pr)
 {
+	if (pr->parent) {
+		close_page_read(pr->parent);
+		xfree(pr->parent);
+	}
+
 	close(pr->fd_pg);
 	close(pr->fd);
 }
 
+static int open_page_read_at(int dfd, int pid, struct page_read *pr);
+
+static int try_open_parent(int dfd, int pid, struct page_read *pr)
+{
+	int pfd;
+	struct page_read *parent = NULL;
+
+	pfd = openat(dfd, CR_PARENT_LINK, O_RDONLY);
+	if (pfd < 0 && errno == ENOENT)
+		goto out;
+
+	parent = xmalloc(sizeof(*parent));
+	if (!parent)
+		goto err_cl;
+
+	if (open_page_read_at(pfd, pid, parent))
+		goto err_free;
+
+	close(pfd);
+out:
+	pr->parent = parent;
+	return 0;
+
+err_free:
+	xfree(parent);
+err_cl:
+	close(pfd);
+	return -1;
+}
+
 static int open_page_read_at(int dfd, int pid, struct page_read *pr)
 {
 	pr->fd = open_image_at(dfd, CR_FD_PAGEMAP, O_RSTR, (long)pid);
@@ -83,10 +181,18 @@ static int open_page_read_at(int dfd, int pid, struct page_read *pr)
 		if (pr->fd_pg < 0)
 			return -1;
 
+		pr->parent = NULL;
 		pr->get_pagemap = get_page_vaddr;
 		pr->put_pagemap = NULL;
 		pr->read_page = read_page;
 	} else {
+		static unsigned ids = 1;
+
+		if (try_open_parent(dfd, pid, pr)) {
+			close(pr->fd);
+			return -1;
+		}
+
 		pr->fd_pg = open_pages_image_at(dfd, O_RSTR, pr->fd);
 		if (pr->fd_pg < 0) {
 			close_page_read(pr);
@@ -96,6 +202,10 @@ static int open_page_read_at(int dfd, int pid, struct page_read *pr)
 		pr->get_pagemap = get_pagemap;
 		pr->put_pagemap = put_pagemap;
 		pr->read_page = read_pagemap_page;
+		pr->id = ids++;
+
+		pr_debug("Opened page read %u (parent %u)\n",
+				pr->id, pr->parent ? pr->parent->id : 0);
 	}
 
 	pr->close = close_page_read;