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: <297f5209-603e-a50d-c27b-8e50d23f86de@collabora.com>
Date:   Tue, 12 Sep 2023 02:41:58 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Boris Brezillon <boris.brezillon@...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 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. This is a fix for a minor bug that existed for
many years and is difficult to trigger in practice, it can wait.

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.

-- 
Best regards,
Dmitry