linux-kernel - Re: [PATCH v2 1/2] drm/gpuvm: add deferred vm

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Message-ID: <20250911135746.1c9cdd4b@fedora>
Date: Thu, 11 Sep 2025 13:57:46 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Matthew Brost
 <matthew.brost@...el.com>, "Thomas Hellström"
 <thomas.hellstrom@...ux.intel.com>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Steven Price <steven.price@....com>,
 Daniel Almeida <daniel.almeida@...labora.com>, Liviu Dudau
 <liviu.dudau@....com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 1/2] drm/gpuvm: add deferred vm_bo cleanup

On Tue, 9 Sep 2025 13:39:39 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:

> 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@...gle.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.

Yep, I think I prefer this version too.