[Intel-gfx] [PATCH] drm/i915: Allocate atomically in execbuf path

[Ben Widawsky]

On Tue, Nov 26, 2013 at 04:55:50PM -0800, Ben Widawsky wrote:
> If we end up calling the shrinker, which in turn requires the OOM
> killer, we may end up infinitely waiting for a process to die if the OOM
> chooses. The case that this prevents occurs in execbuf. The forked
> variants of gem_evict_everything is a good way to hit it. This is
> exacerbated by Daniel's recent patch to give OOM precedence to the GEM
> tests.
> 
> It's a twisted form of a deadlock.
> 
> What occurs is the following (assume just 2 procs)
> 1. proc A gets to execbuf while out of memory, gets struct_mutex.
> 2. OOM killer comes in and chooses proc B
> 3. proc B closes it's fds, which requires struct mutex, blocks
> 4, OOM killer waits for B to die before killing another process (this
> part is speculative)
> 

It appears that by itself, this patch is insufficient to prevent the
failure when run in piglit. Before I go on a wild goose chase of finding
all allocations, maybe I'll give people a chance to chime in. The
symptom is the same always, OOM, procs can't die because struct_mutex is
held.

> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index fb2d548..a60894d 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1842,12 +1842,12 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
>  	BUG_ON(obj->base.read_domains & I915_GEM_GPU_DOMAINS);
>  	BUG_ON(obj->base.write_domain & I915_GEM_GPU_DOMAINS);
>  
> -	st = kmalloc(sizeof(*st), GFP_KERNEL);
> +	st = kmalloc(sizeof(*st), GFP_NOWAIT);
>  	if (st == NULL)
>  		return -ENOMEM;
>  
>  	page_count = obj->base.size / PAGE_SIZE;
> -	if (sg_alloc_table(st, page_count, GFP_KERNEL)) {
> +	if (sg_alloc_table(st, page_count, GFP_NOWAIT)) {
>  		kfree(st);
>  		return -ENOMEM;
>  	}
> -- 
> 1.8.4.2
> 

-- 
Ben Widawsky, Intel Open Source Technology Center