Re: [PATCH drm-next v2 04/16] maple_tree: add flag MT_FLAGS_LOCK_NONE

[Matthew Wilcox]

On Fri, Feb 17, 2023 at 02:44:10PM +0100, Danilo Krummrich wrote:
> Generic components making use of the maple tree (such as the
> DRM GPUVA Manager) delegate the responsibility of ensuring mutual
> exclusion to their users.
> 
> While such components could inherit the concept of an external lock,
> some users might just serialize the access to the component and hence to
> the internal maple tree.
> 
> In order to allow such use cases, add a new flag MT_FLAGS_LOCK_NONE to
> indicate not to do any internal lockdep checks.

I'm really against this change.

First, we really should check that users have their locking right.
It's bitten us so many times when they get it wrong.

Second, having a lock allows us to defragment the slab cache.  The
patches to do that haven't gone anywhere recently, but if we drop the
requirement now, we'll never be able to compact ranges of memory that
have slabs allocated to them.