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

[Dmitry Osipenko]

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