[PATCH] Disallow swapoff if there is insufficient memory

From: Hugh Dickins <hugh@veritas.com>

First of three small "stop swapoff" patches based on 2.5.67-mm3:

stop swapoff 1/3 vm_enough_memory?

Before embarking upon swapoff, check vm_enough_memory.  Mainly
for consistency in the overcommit_memory 2 (strict accounting) case:
fail with -ENOMEM if it wouldn't let the amount removed be committed.

Will always succeed in the overcommit_memory 1 case, as it should in
root-shoot-foot mode.  In the overcommit_memory 0 case, well, I don't
care much either way, so opted for the simplest code: no special case.
Which means it could now fail at the start; but that's unlikely (case 0
is over-generous) and only when it would have got stuck later on anyway.

mm/swapfile.c

@@ -7,6 +7,7 @@
 
 #include <linux/config.h>
 #include <linux/mm.h>
+#include <linux/mman.h>
 #include <linux/slab.h>
 #include <linux/kernel_stat.h>
 #include <linux/swap.h>
@@ -1028,12 +1029,18 @@ asmlinkage long sys_swapoff(const char __user * specialfile)
 		}
 		prev = type;
 	}
-	err = -EINVAL;
 	if (type < 0) {
+		err = -EINVAL;
+		swap_list_unlock();
+		goto out_dput;
+	}
+	if (vm_enough_memory(p->pages))
+		vm_unacct_memory(p->pages);
+	else {
+		err = -ENOMEM;
 		swap_list_unlock();
 		goto out_dput;
 	}
-
 	if (prev < 0) {
 		swap_list.head = p->next;
 	} else {