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

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Message-Id: <DCNAE3CJMEJ0.JH1F0MJABXQI@kernel.org>
Date: Mon, 08 Sep 2025 10:47:25 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Boris Brezillon" <boris.brezillon@...labora.com>, "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 1/2] drm/gpuvm: add deferred vm_bo cleanup

On Mon Sep 8, 2025 at 10:26 AM CEST, Alice Ryhl wrote:
> On Mon, Sep 08, 2025 at 09:11:40AM +0200, Boris Brezillon wrote:
>> Hi Alice,
>> 
>> On Sun, 7 Sep 2025 11:39:41 +0000
>> Alice Ryhl <aliceryhl@...gle.com> wrote:
>> 
>> > Yeah I guess we could have unlink remove the gpuva, but then allow the
>> > end-user to attach the gpuva to a list of gpuvas to kfree deferred. That
>> > way, the drm_gpuva_unlink() is not deferred but any resources it has can
>> > be.
>> 
>> This ^.
>> 
>> > 
>> > Of course, this approach also makes deferred gpuva cleanup somewhat
>> > orthogonal to this patch.
>> 
>> Well, yes and no, because if you go for gpuva deferred cleanup, you
>> don't really need the fancy kref_put() you have in this patch, it's
>> just a regular vm_bo_put() that's called in the deferred gpuva path on
>> the vm_bo attached to the gpuva being released.
>
> Ok, so what you suggest is that on gpuva_unlink() we remove the gpuva
> from the vm_bo's list, but then instead of putting the vm_bo's refcount,
> we add the gpuva to a list, and in the deferred cleanup codepath we
> iterate gpuvas and drop vm_bo refcounts *at that point*. Is that
> understood correctly?

It has to be a special unlink function though, since otherwise

	drm_gpuva_link();
	drm_gpuva_unlink();
	drm_gpuva_link();
	drm_gpuva_unlink();

leaks the VM_BO. Sounds a bit messy, but my concern is really about the below:

> That means we don't immediately remove the vm_bo from the gem.gpuva
> list, but the gpuva list in the vm_bo will be empty. I guess you already
> have to handle such vm_bos anyway since you can already have an empty
> vm_bo in between vm_bo_obtain() and the first call to gpuva_link().
>
> One disadvantage is that we might end up preparing or unevicting a GEM
> object that doesn't have any VAs left, which the current approach
> avoids.

Yeah, we really want to avoid that.