[PATCH v2 1/2] mmap locking API: Order lock of nascent mm outside lock of live mm

[Johannes Berg]

On Wed, 2020-10-07 at 00:54 +0200, Jann Horn wrote:
> Until now, the mmap lock of the nascent mm was ordered inside the mmap lock
> of the old mm (in dup_mmap() and in UML's activate_mm()).
> A following patch will change the exec path to very broadly lock the
> nascent mm, but fine-grained locking should still work at the same time for
> the old mm.
> 
> In particular, mmap locking calls are hidden behind the copy_from_user()
> calls and such that are reached through functions like copy_strings() -
> when a page fault occurs on a userspace memory access, the mmap lock
> will be taken.
> 
> To do this in a way that lockdep is happy about, let's turn around the lock
> ordering in both places that currently nest the locks.
> Since SINGLE_DEPTH_NESTING is normally used for the inner nesting layer,
> make up our own lock subclass MMAP_LOCK_SUBCLASS_NASCENT and use that
> instead.
> 
> The added locking calls in exec_mmap() are temporary; the following patch
> will move the locking out of exec_mmap().
> 
> Signed-off-by: Jann Horn <jannh at google.com>
> ---
>  arch/um/include/asm/mmu_context.h |  3 +--
>  fs/exec.c                         |  4 ++++
>  include/linux/mmap_lock.h         | 23 +++++++++++++++++++++--
>  kernel/fork.c                     |  7 ++-----
>  4 files changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/um/include/asm/mmu_context.h b/arch/um/include/asm/mmu_context.h
> index 17ddd4edf875..c13bc5150607 100644
> --- a/arch/um/include/asm/mmu_context.h
> +++ b/arch/um/include/asm/mmu_context.h
> @@ -48,9 +48,8 @@ static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
>  	 * when the new ->mm is used for the first time.
>  	 */
>  	__switch_mm(&new->context.id);
> -	mmap_write_lock_nested(new, SINGLE_DEPTH_NESTING);
> +	mmap_assert_write_locked(new);
>  	uml_setup_stubs(new);
> -	mmap_write_unlock(new);
>  }

FWIW, this was I believe causing lockdep issues.

I think I had previously determined that this was pointless, since it's
still nascent and cannot be used yet? But I didn't really know for sure,
and this patch was never applied:

https://patchwork.ozlabs.org/project/linux-um/patch/20200604133752.397dedea0758.I7a24aaa26794eb3fa432003c1bf55cbb816489e2@changeid/

I guess your patches will also fix the lockdep complaints in UML in this
area, I hope I'll be able to test it soon.

johannes