Re: [PATCH v2 1/2] drm/gpuvm: add deferred vm_bo cleanup

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Sep 09, 2025 at 01:36:22PM +0000, Alice Ryhl wrote:
> When using GPUVM in immediate mode, it is necessary to call
> drm_gpuvm_unlink() from the fence signalling critical path. However,
> unlink may call drm_gpuvm_bo_put(), which causes some challenges:
> 
> 1. drm_gpuvm_bo_put() often requires you to take resv locks, which you
>    can't do from the fence signalling critical path.
> 2. drm_gpuvm_bo_put() calls drm_gem_object_put(), which is often going
>    to be unsafe to call from the fence signalling critical path.
> 
> To solve these issues, add a deferred version of drm_gpuvm_unlink() that
> adds the vm_bo to a deferred cleanup list, and then clean it up later.
> 
> The new methods take the GEMs GPUVA lock internally rather than letting
> the caller do it because it also needs to perform an operation after
> releasing the mutex again. This is to prevent freeing the GEM while
> holding the mutex (more info as comments in the patch). This means that
> the new methods can only be used with DRM_GPUVM_IMMEDIATE_MODE.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

I'm not sure if we came to a conclusion on the gpuva defer stuff on v1,
but I found a less confusing way to implement the locking. Please check
it out.

Alice