parasite: page dump -- Map pages huge bitmap inplace

In case if there is the VMA which has a huge size (4,8,16 G)
then the prealoocated bitmap space won't be enough to track
all pages. Thus we call for system help invoking mmap() call
with bitmap length needed.

We simply assume that if such big VMA present the node has enough
memory to provide us space for pages huge bitmap.
Reported-by: Adrian Reber <adrian@lisas.de>
Tested-by: Adrian Reber <adrian@lisas.de>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

pie/parasite.c

@@ -137,6 +137,7 @@ static int dump_pages(struct parasite_dump_pages_args *args)
 {
 	unsigned long nrpages, pfn, length;
 	unsigned long prot_old, prot_new;
+	bool bigmap = false;
 	u64 *map, off;
 	int ret = -1;
 
@@ -154,9 +155,20 @@ static int dump_pages(struct parasite_dump_pages_args *args)
 	 */
 	map = brk_alloc(length);
 	if (!map) {
+		/*
+		 * Lets try allocate the bitmap inplace. If the VMA
+		 * is that big we assume the node has enough physical
+		 * memory.
+		 */
+		map = (u64 *)sys_mmap(NULL, length,
+				      PROT_READ | PROT_WRITE,
+				      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+		if ((long)(void *)map > TASK_SIZE) {
 			ret = -ENOMEM;
 			goto err;
 		}
+		bigmap = true;
+	}
 
 	off = pfn * sizeof(*map);
 	off = sys_lseek(fd_pagemap, off, SEEK_SET);
@@ -226,7 +238,10 @@ static int dump_pages(struct parasite_dump_pages_args *args)
 
 	ret = 0;
 err_free:
+	if (!bigmap)
 		brk_free(length);
+	else
+		sys_munmap(map, length);
 err:
 	return ret;
 }