Re: [PATCH] drm/i915: Allocate atomically in execbuf path

[Ben Widawsky <ben@bwidawsk.net>]

On Tue, Nov 26, 2013 at 05:10:38PM -0800, Ben Widawsky wrote:
> 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.
> 

too late on the goose chase. vma allocation was the missing bit.

> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > Signed-off-by: Ben Widawsky <ben@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
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-- 
Ben Widawsky, Intel Open Source Technology Center