[Intel-gfx] [PATCH 09/11] drm/i915: Migrate stolen objects before hibernation

[Chris Wilson]

On Thu, Jan 14, 2016 at 11:46:45AM +0530, ankitprasad.r.sharma at intel.com wrote:
> From: Chris Wilson <chris at chris-wilson.co.uk>
> 
> Ville reminded us that stolen memory is not preserved across
> hibernation, and a result of this was that context objects now being
> allocated from stolen were being corrupted on S4 and promptly hanging
> the GPU on resume.
> 
> We want to utilise stolen for as much as possible (nothing else will use
> that wasted memory otherwise), so we need a strategy for handling
> general objects allocated from stolen and hibernation. A simple solution
> is to do a CPU copy through the GTT of the stolen object into a fresh
> shmemfs backing store and thenceforth treat it as a normal objects. This
> can be refined in future to either use a GPU copy to avoid the slow
> uncached reads (though it's hibernation!) and recreate stolen objects
> upon resume/first-use. For now, a simple approach should suffice for
> testing the object migration.
> 
> v2:
> Swap PTE for pinned bindings over to the shmemfs. This adds a
> complicated dance, but is required as many stolen objects are likely to
> be pinned for use by the hardware. Swapping the PTEs should not result
> in externally visible behaviour, as each PTE update should be atomic and
> the two pages identical. (danvet)
> 
> safe-by-default, or the principle of least surprise. We need a new flag
> to mark objects that we can wilfully discard and recreate across
> hibernation. (danvet)
> 
> Just use the global_list rather than invent a new stolen_list. This is
> the slowpath hibernate and so adding a new list and the associated
> complexity isn't worth it.
> 
> v3: Rebased on drm-intel-nightly (Ankit)
> 
> v4: Use insert_page to map stolen memory backed pages for migration to
> shmem (Chris)
> 
> v5: Acquire mutex lock while copying stolen buffer objects to shmem (Chris)
> 
> v6: Handled file leak, Splitted object migration function, added kerneldoc
> for migrate_stolen_to_shmemfs() function (Tvrtko)
> Use i915 wrapper function for drm_mm_insert_node_in_range()
> 
> v7: Keep the object in cpu domain after get_pages, remove the object from
> the unbound list only when marked PURGED, Corrected split of object migration
> function (Chris)
> 
> v8: Split i915_gem_freeze(), removed redundant use of barrier, corrected
> use of set_to_cpu_domain() (Chris)
> 
> v9: Replaced WARN_ON by BUG_ON and added a comment explaining it (Tvrtko)

I read Daniel's comment as this should be BUG_ON. It is machine death. A
way for userspace to engineer writes to unowned pages.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre