Commit 2f60f8d3 authored Aug 04, 2005 by Simon Derr Committed by Linus Torvalds Aug 04, 2005

[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>

parent b68e9f85

mm/mmap.c

+5 −1

Original line number	Diff line number	Diff line
		@@ -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);

mm/nommu.c

+5 −1

Original line number	Diff line number	Diff line
		@@ -1167,7 +1167,11 @@ 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);