drm/i915/userptr: Hold mmref whilst calling get-user-pages
commit db9f9203 upstream. Holding a reference to the containing task_struct is not sufficient to prevent the mm_struct from being reaped under memory pressure. If this happens whilst we are calling get_user_pages(), explosions erupt - sometimes an immediate GPF, sometimes page flag corruption. To prevent the target mm from being reaped as we are reading from it, acquire a reference before we begin. Testcase: igt/gem_shrink/*userptr Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: Michał Winiarski <michal.winiarski@intel.com> Reviewed-by: Michał Winiarski <michal.winiarski@intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/1459864801-28606-2-git-send-email-chris@chris-wilson.co.uk (cherry picked from commit 40313f0c) Signed-off-by: Jani Nikula <jani.nikula@intel.com> [bwh: Backported to 3.16: - s/get_user_pages_remote/get_user_pages/ - s/npages/num_pages/] Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
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