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

[Matthew Wilcox]

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.

ie I would drop these lines:

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