linux-kernel - Re: [PATCH v3 1/2] drm/gpuvm: add deferred vm

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Message-ID: <20251006133832.6d385b23@fedora>
Date: Mon, 6 Oct 2025 13:38:32 +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 v3 1/2] drm/gpuvm: add deferred vm_bo cleanup

On Mon, 6 Oct 2025 13:30:59 +0200
Alice Ryhl <aliceryhl@...gle.com> wrote:

> On Wed, Oct 1, 2025 at 12:41 PM Alice Ryhl <aliceryhl@...gle.com> 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.
> >
> > Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
> > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>  
> 
> In this version, I got rid of the kref_put_mutex() usage, but I
> realized that maybe we should bring it back. With the current code,
> it's actually possible to observe a zombie vm_bo in the GEM's list
> because we don't drop the refcount while holding the mutex.

Alright, let's get back to the kref_put_mutex() approach then.