The bug is already reported and closed in https://bugreports.qt.io/browse/QTBUG-73084 and https://bugreports.qt.io/browse/QTBUG-83663 but even with all relevant patches I am still getting the same kernel warnings about pages still in use whenever I stop a video before it is finished and play another one.

The patches (https://codereview.qt-project.org/c/qt/qtmultimedia/+/273602, https://codereview.qt-project.org/c/qt/qtmultimedia/+/298634)  seem to rely on checking if the buffer address is in physical memory or not. But in my tests the buffer was never in physical memory, therefore the patch does nothing.

The only way I can get rid of the kernel warnings is by forcing mMappable to false in 
QSGVivanteVideoMaterial, therefore the alternative "copy" path is taken.