swap: warn when a swap area overflows the maximum size

It is possible to swapon a swap area that is too big for the pte width
to handle.

Presently this failure happens silently.

Instead, emit a diagnostic to warn the user.

Testing results, root prompt commands and kernel log messages:

# lvresize /dev/system/swap --size 16G
# mkswap /dev/system/swap
# swapon /dev/system/swap

Jul  7 04:27:22 warfang kernel: Adding 16777212k swap
on /dev/mapper/system-swap.  Priority:-1 extents:1 across:16777212k

# lvresize /dev/system/swap --size 64G
# mkswap /dev/system/swap
# swapon /dev/system/swap

Jul  7 04:27:22 warfang kernel: Truncating oversized swap area, only
using 33554432k out of 67108860k
Jul  7 04:27:22 warfang kernel: Adding 33554428k swap
on /dev/mapper/system-swap.  Priority:-1 extents:1 across:33554428k

[akpm@linux-foundation.org: fix warning]
Signed-off-by: Raymond Jennings <shentino@gmail.com>
Acked-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -1926,6 +1926,7 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
 	int i;
 	unsigned long maxpages;
 	unsigned long swapfilepages;
+	unsigned long last_page;
 
 	if (memcmp("SWAPSPACE2", swap_header->magic.magic, 10)) {
 		printk(KERN_ERR "Unable to find swap-space signature\n");
@@ -1968,8 +1969,15 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
 	 */
 	maxpages = swp_offset(pte_to_swp_entry(
 			swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
-	if (maxpages > swap_header->info.last_page) {
-		maxpages = swap_header->info.last_page + 1;
+	last_page = swap_header->info.last_page;
+	if (last_page > maxpages) {
+		printk(KERN_WARNING
+			"Truncating oversized swap area, only using %luk out of %luk\n",
+			maxpages << (PAGE_SHIFT - 10),
+			last_page << (PAGE_SHIFT - 10));
+	}
+	if (maxpages > last_page) {
+		maxpages = last_page + 1;
 		/* p->max is an unsigned int: don't overflow it */
 		if ((unsigned int)maxpages == 0)
 			maxpages = UINT_MAX;