[PATCH] __vm_enough_memory() signedness fix

We have found what seems to be a small bug in __vm_enough_memory() when
sysctl_overcommit_memory is set to OVERCOMMIT_NEVER.

When this bug occurs the systems fails to boot, with /sbin/init whining
about fork() returning ENOMEM.

We hunted down the problem to this:

The deferred update mecanism used in vm_acct_memory(), on a SMP system,
allows the vm_committed_space counter to have a negative value.

This should not be a problem since this counter is known to be inaccurate.

But in __vm_enough_memory() this counter is compared to the `allowed'
variable, which is an unsigned long.  This comparison is broken since it
will consider the negative values of vm_committed_space to be huge positive
values, resulting in a memory allocation failure.

Signed-off-by: <Jean-Marc.Saffroy@ext.bull.net>
Signed-off-by: <Simon.Derr@bull.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index da3fa90a0aae..404319477e71 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -143,7 +143,11 @@ int __vm_enough_memory(long pages, int cap_sys_admin)
            leave 3% of the size of this process for other processes */
         allowed -= current->mm->total_vm / 32;
 
-        if (atomic_read(&vm_committed_space) < allowed)
+        /*
+         * cast `allowed' as a signed long because vm_committed_space
+         * sometimes has a negative value
+         */
+        if (atomic_read(&vm_committed_space) < (long)allowed)
                 return 0;
 
         vm_unacct_memory(pages);