linux-kernel - Re: [PATCH v16 02/20] drm/shmem-helper: Use flag for tracking page count bumped by get_pages

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Message-ID: <20230912090710.72e998e6@collabora.com>
Date:   Tue, 12 Sep 2023 09:07:10 +0200
From:   Boris Brezillon <boris.brezillon@...labora.com>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc:     David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v16 02/20] drm/shmem-helper: Use flag for tracking page
 count bumped by get_pages_sgt()

On Tue, 12 Sep 2023 02:41:58 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:

> On 9/5/23 10:40, Boris Brezillon wrote:
> > On Sun,  3 Sep 2023 20:07:18 +0300
> > Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:
> >   
> >> Use separate flag for tracking page count bumped by shmem->sgt to avoid
> >> imbalanced page counter during of drm_gem_shmem_free() time. It's fragile
> >> to assume that populated shmem->pages at a freeing time means that the
> >> count was bumped by drm_gem_shmem_get_pages_sgt(), using a flag removes
> >> the ambiguity.
> >>
> >> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> >> ---
> >>  drivers/gpu/drm/drm_gem_shmem_helper.c | 11 ++++++++++-
> >>  drivers/gpu/drm/lima/lima_gem.c        |  1 +
> >>  include/drm/drm_gem_shmem_helper.h     |  7 +++++++
> >>  3 files changed, 18 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> index 6693d4061ca1..848435e08eb2 100644
> >> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> @@ -152,8 +152,10 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
> >>  			sg_free_table(shmem->sgt);
> >>  			kfree(shmem->sgt);
> >>  		}
> >> -		if (shmem->pages)
> >> +		if (shmem->pages) {
> >>  			drm_gem_shmem_put_pages(shmem);
> >> +			drm_WARN_ON(obj->dev, !shmem->got_pages_sgt);
> >> +		}  
> > 
> > Already mentioned in v15, but I keep thinking the following:
> > 
> > 		if (shmem->sgt) {
> > 			// existing code in the preceding
> > 			// if (shmem->sgt) branch
> > 			...
> > 
> > 			/*
> > 			 * Release the implicit pages ref taken in
> > 			 * drm_gem_shmem_get_pages_sgt_locked().
> > 			 */
> > 			drm_gem_shmem_put_pages(shmem);
> > 		}
> > 
> > does exactly the same without requiring the addition of a new field.  
> 
> I'll factor out these "flag" patches into separate patchset since they
> cause too many questions.

So patch 1 and 2 in this series right?

> This is a fix for a minor bug that existed for
> many years

I'm not denying the fact there's a bug, but I'm convinced this can be
fixed without adding new flags. If there's something wrong with the
suggested solution, I'd be interested to hear more about it.

> and is difficult to trigger in practice, it can wait.

Triggering it with a real fault is hard, but you can manually fake
errors at any point in the initialization process and check what
happens.

> 
> For now will be better to focus on finishing and landing the refcnt and
> shrinker patches, the rest of drm-shmem core improvements can be done
> afterwards.
> 

Sounds good to me.