linux-kernel - Re: [PATCH v15 02/23] drm/shmem-helper: Use flag for tracking page count bumped by get_pages

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Date:   Sat, 2 Sep 2023 21:28:21 +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>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org,
        intel-gfx@...ts.freedesktop.org
Subject: Re: [PATCH v15 02/23] drm/shmem-helper: Use flag for tracking page
 count bumped by get_pages_sgt()

On 8/28/23 13:55, Boris Brezillon wrote:
> On Sun, 27 Aug 2023 20:54:28 +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 | 3 ++-
>>  drivers/gpu/drm/lima/lima_gem.c        | 1 +
>>  include/drm/drm_gem_shmem_helper.h     | 7 +++++++
>>  3 files changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index 78d9cf2355a5..db20b9123891 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -152,7 +152,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>>  			sg_free_table(shmem->sgt);
>>  			kfree(shmem->sgt);
>>  		}
>> -		if (shmem->pages)
>> +		if (shmem->got_sgt)
>>  			drm_gem_shmem_put_pages(shmem);
> 
> Can't we just move this drm_gem_shmem_put_pages() call in the
> if (shmem->sgt) block?

As you've seen in patch #1, the shmem->sgt may belong to imported dmabuf
and pages aren't referenced in this case.

I agree that the freeing code is confusing. The flags make it a better,
not ideal. Though, the flags+comments solution is good enough to me.
Please let me know if you have more suggestions, otherwise I'll add
comment to the code and keep this patch for v16.

BTW, I realized that the new flag wasn't placed properly in the Lima
driver, causing unbalanced page count in the error path. Will correct it
in v16.

-- 
Best regards,
Dmitry