Re: __vm_enough_memory(), OVERCOMMIT_NEVER, current->mm, kernel thread

From: hooanon05
Date: Tue Oct 21 2008 - 19:11:19 EST

Alan Cox:
> Calling into the file system code assuming that current->mm is
> NULL isn't safe and hasn't been for a very long time since someone added
> the 3% hack.

I guess
- people don't care overcommit and leave it as default, so they don't
meet the problem
- people who cares overcommit has rich memory, and they don't meet the
problem too.

> The shmem case is actually a bit special so my thoughts are:
> Make security_vm_enough_memory() WARN() if current->mm = NULL
> Make security_vm_enough_memory_mm() WARN() if the passed mm = NULL
> Add security_vm_enough_memory_fs() which does not do the warning test
> All would still call security->ops->vm_enough_memory and then
> __vm_enough_memory() would skip the 3% adjustment when the passed mm was
> Does that sound sensible ?

In your first option, write() to the exported tmpfs will produce the
warning on nfs server even if much memory is left. I don't think it is a
good idea.
I'd suggest to make __vm_enough_memory() would skip the 3% adjustment

--- /src/linux-2.6/linux-2.6.27/mm/mmap.c 2008-10-10 07:13:53.000000000 +0900
+++ /tmp/mmap.c 2008-10-22 08:07:09.000000000 +0900
@@ -173,9 +173,10 @@
allowed -= allowed / 32;
allowed += total_swap_pages;

- /* Don't let a single process grow too big:
+ /* Don't let a single user process grow too big:
leave 3% of the size of this process for other processes */
- allowed -= mm->total_vm / 32;
+ if (mm)
+ allowed -= mm->total_vm / 32;

* cast `allowed' as a signed long because vm_committed_space

Junjiro R. Okajima
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at