Re: [PATCH] mm: don't overwrite mm->def_flags in do_mlockall()

Andrew Morton
2013-02-06T20:51:09Z

On Wed, 6 Feb 2013 16:49:34 +0100
Gerald Schaefer <gerald.schaefer@de.ibm.com> wrote:

> With commit 8e72033 "thp: make MADV_HUGEPAGE check for mm->def_flags"
> the VM_NOHUGEPAGE flag may be set on s390 in mm->def_flags for certain
> processes, to prevent future thp mappings. This would be overwritten
> by do_mlockall(), which sets it back to 0 with an optional VM_LOCKED
> flag set.
>
> To fix this, instead of overwriting mm->def_flags in do_mlockall(),
> only the VM_LOCKED flag should be set or cleared.

What are the user-visible effects here? Looking at the 274023da1e8
changelog, I'm guessing that it might be pretty nasty - kvm breakage?

> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -517,11 +517,11 @@ SYSCALL_DEFINE2(munlock, unsigned long, start, size_t, len)
> static int do_mlockall(int flags)
> {
> struct vm_area_struct * vma, * prev = NULL;
> - unsigned int def_flags = 0;
>
> if (flags & MCL_FUTURE)
> - def_flags = VM_LOCKED;
> - current->mm->def_flags = def_flags;
> + current->mm->def_flags |= VM_LOCKED;
> + else
> + current->mm->def_flags &= ~VM_LOCKED;
> if (flags == MCL_FUTURE)
> goto out;

Michal sent an equivalent patch last month: