drm/i915: Return immediately if trylock fails for direct-reclaim
Ignore trying to shrink from i915 if we fail to acquire the struct_mutex in the shrinker while performing direct-reclaim. The trade-off being (much) lower latency for non-i915 clients at an increased risk of being unable to obtain a page from direct-reclaim without hitting the oom-notifier. The proviso being that we still keep trying to hard obtain the lock for kswapd so that we can reap under heavy memory pressure. v2: Taint all mutexes taken within the shrinker with the struct_mutex subclass as an early warning system, and drop I915_SHRINK_ACTIVE from vmap to reduce the number of dangerous paths. We also have to drop I915_SHRINK_ACTIVE from oom-notifier to be able to make the same claim that ACTIVE is only used from outside context, which fits in with a longer strategy of avoiding stalls due to scanning active during shrinking. The danger in using the subclass struct_mutex is that we declare ourselves more knowledgable than lockdep and deprive ourselves of automatic coverage. Instead, we require ourselves to mark up any mutex taken inside the shrinker in order to detect lock-inversion, and if we miss any we are doomed to a deadlock at the worst possible moment. Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20190107115509.12523-1-chris@chris-wilson.co.uk
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