Re: [PATCH] mm/memory.c: do_fault: avoid usage of stale vm_area_struct

[Jan Stancek]

----- Original Message -----
> On Sat, Mar 02, 2019 at 04:11:26PM +0100, Jan Stancek wrote:
> > Problem is that "vmf->vma" used in do_fault() can become stale.
> > Because mmap_sem may be released, other threads can come in,
> > call munmap() and cause "vma" be returned to kmem cache, and
> > get zeroed/re-initialized and re-used:
> 
> > This patch pins mm_struct and stores its value, to avoid using
> > potentially stale "vma" when calling pte_free().
> 
> OK, we need to cache the mm_struct, but why do we need the extra atomic op?
> There's surely no way the mm can be freed while the thread is in the middle
> of handling a fault.

You're right, I was needlessly paranoid.

> 
> ie I would drop these lines:

I'll send v2.

Thanks,
Jan

> 
> > +	mmgrab(vm_mm);
> > +
> ...
> > +
> > +	mmdrop(vm_mm);
> > +
>