• Chris Wilson's avatar
    drm/i915: Pull i915_vma_pin under the vm->mutex · 2850748e
    Chris Wilson authored
    Replace the struct_mutex requirement for pinning the i915_vma with the
    local vm->mutex instead. Note that the vm->mutex is tainted by the
    shrinker (we require unbinding from inside fs-reclaim) and so we cannot
    allocate while holding that mutex. Instead we have to preallocate
    workers to do allocate and apply the PTE updates after we have we
    reserved their slot in the drm_mm (using fences to order the PTE writes
    with the GPU work and with later unbind).
    
    In adding the asynchronous vma binding, one subtle requirement is to
    avoid coupling the binding fence into the backing object->resv. That is
    the asynchronous binding only applies to the vma timeline itself and not
    to the pages as that is a more global timeline (the binding of one vma
    does not need to be ordered with another vma, nor does the implicit GEM
    fencing depend on a vma, only on writes to the backing store). Keeping
    the vma binding distinct from the backing store timelines is verified by
    a number of async gem_exec_fence and gem_exec_schedule tests. The way we
    do this is quite simple, we keep the fence for the vma binding separate
    and only wait on it as required, and never add it to the obj->resv
    itself.
    
    Another consequence in reducing the locking around the vma is the
    destruction of the vma is no longer globally serialised by struct_mutex.
    A natural solution would be to add a kref to i915_vma, but that requires
    decoupling the reference cycles, possibly by introducing a new
    i915_mm_pages object that is own by both obj->mm and vma->pages.
    However, we have not taken that route due to the overshadowing lmem/ttm
    discussions, and instead play a series of complicated games with
    trylocks to (hopefully) ensure that only one destruction path is called!
    
    v2: Add some commentary, and some helpers to reduce patch churn.
    Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
    Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
    Reviewed-by: default avatarTvrtko Ursulin <tvrtko.ursulin@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20191004134015.13204-4-chris@chris-wilson.co.uk
    2850748e
i915_gem_shrinker.c 14.9 KB