linux-kernel - Re: [PATCH v16 15/20] drm/shmem-helper: Add memory shrinker

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Message-ID: <26f7ba6d-3520-0311-35e2-ef5706a98232@collabora.com>
Date:   Wed, 13 Sep 2023 03:56:14 +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 15/20] drm/shmem-helper: Add memory shrinker

On 9/5/23 11:03, Boris Brezillon wrote:
>>                * But
>> +		 * acquiring the obj lock in drm_gem_shmem_release_pages_locked() can
>> +		 * cause a locking order inversion between reservation_ww_class_mutex
>> +		 * and fs_reclaim.
>> +		 *
>> +		 * This deadlock is not actually possible, because no one should
>> +		 * be already holding the lock when drm_gem_shmem_free() is called.
>> +		 * Unfortunately lockdep is not aware of this detail.  So when the
>> +		 * refcount drops to zero, don't touch the reservation lock.
>> +		 */
>> +		if (shmem->got_pages_sgt &&
>> +		    refcount_dec_and_test(&shmem->pages_use_count)) {
>> +			drm_gem_shmem_do_release_pages_locked(shmem);
>> +			shmem->got_pages_sgt = false;
>>  		}
> Leaking memory is the right thing to do if pages_use_count > 1 (it's
> better to leak than having someone access memory it no longer owns), but
> I think it's worth mentioning in the above comment.

It's unlikely that it will be only a leak without a following up
use-after-free. Neither is acceptable.

The drm_gem_shmem_free() could be changed such that kernel won't blow up
on a refcnt bug, but that's not worthwhile doing because drivers
shouldn't have silly bugs.

-- 
Best regards,
Dmitry