linux-kernel - Re: [PATCH v15 16/23] drm/shmem-helper: Use kref for vmap_use

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Message-ID: <9844875a-ee18-85ff-11bd-84bdb2333f6c@collabora.com>
Date:   Sat, 2 Sep 2023 23:22:25 +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 16/23] drm/shmem-helper: Use kref for vmap_use_count

On 8/28/23 13:00, Boris Brezillon wrote:
> On Sun, 27 Aug 2023 20:54:42 +0300
> Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:
> 
>> Use kref helper for vmap_use_count to make refcounting consistent with
>> pages_use_count and pages_pin_count that use kref. This will allow to
>> optimize unlocked vmappings by skipping reservation locking if refcnt > 1.
> 
> The core is taking the resv lock before calling ->v[un]map(), so
> switching to a kref sounds a bit premature/useless, unless there are
> plans to delegate the locking to the drivers. The only thing it brings
> is standard overflow/underflow checks. Not really sure it's worth
> transitioning to a kref for this field until we have a real use case.

The overflow checks worth transitioning. I'll mention them in the commit
message for v16.

-- 
Best regards,
Dmitry