mm: catch memory commitment underflow

Print a warning (if CONFIG_DEBUG_VM=y) when memory commitment becomes
too negative.

This shouldn't happen any more - the previous two patches fixed the
committed_as underflow issues.

[[email protected]: use VM_WARN_ONCE, per Dave]
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Dave Hansen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/mmap.c

@@ -31,6 +31,7 @@
 #include <linux/mempolicy.h>
 #include <linux/rmap.h>
 #include <linux/mmu_notifier.h>
+#include <linux/mmdebug.h>
 #include <linux/perf_event.h>
 #include <linux/audit.h>
 #include <linux/khugepaged.h>
@@ -134,6 +135,10 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
 {
 	unsigned long free, allowed, reserve;
 
+	VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
+			-(s64)vm_committed_as_batch * num_online_cpus(),
+			"memory commitment underflow");
+
 	vm_acct_memory(pages);
 
 	/*