Re: [PATCH] drm/i915/gem: Suppress oom warning in favour of ENOMEM to userspace

[Nirmoy Das <nirmoy.das@xxxxxxxxx>]

Hi Rodrigo,

On 6/26/2024 5:24 PM, Rodrigo Vivi wrote:

> On Wed, Jun 26, 2024 at 04:33:18PM +0200, Nirmoy Das wrote:
> > We report object allocation failures to userspace with ENOMEM
> > so add __GFP_NOWARN to remove superfluous oom warnings.
> >
> > Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/4936
> > Cc: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>
> > Signed-off-by: Nirmoy Das <nirmoy.das@xxxxxxxxx>
> > ---
> >  drivers/gpu/drm/i915/i915_scatterlist.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_scatterlist.c b/drivers/gpu/drm/i915/i915_scatterlist.c
> > index e93d2538f298..4d830740946d 100644
> > --- a/drivers/gpu/drm/i915/i915_scatterlist.c
> > +++ b/drivers/gpu/drm/i915/i915_scatterlist.c
> > @@ -90,7 +90,7 @@ struct i915_refct_sgt *i915_rsgt_from_mm_node(const struct drm_mm_node *node,
> >  
> >  	GEM_BUG_ON(!max_segment);
> >  
> > -	rsgt = kmalloc(sizeof(*rsgt), GFP_KERNEL);
> > +	rsgt = kmalloc(sizeof(*rsgt), GFP_KERNEL | __GFP_NOWARN);
> >  	if (!rsgt)
> >  		return ERR_PTR(-ENOMEM);
> >       
> is it really safe?
> I don't believe we can guarantee a good fallback plan here if allocation fails.
> __i915_refct_sgt_init
> might end up in a null dereference, no?!
>

Kernel is now returning  ENOMEM and also throwing a oom warning stack. With __GFP_NOWARN

the oom warning stack won't be there in the dmesg but userspace will still get ENOMEM as expected.

Let me know if got your question correctly.

Regards,

Nirmoy

> >  
> > @@ -104,7 +104,7 @@ struct i915_refct_sgt *i915_rsgt_from_mm_node(const struct drm_mm_node *node,
> >  	}
> >  
> >  	if (sg_alloc_table(st, DIV_ROUND_UP_ULL(node->size, segment_pages),
> > -			   GFP_KERNEL)) {
> > +			   GFP_KERNEL | __GFP_NOWARN)) {
> >  		i915_refct_sgt_put(rsgt);
> >  		return ERR_PTR(-ENOMEM);
> >  	}
> > @@ -178,7 +178,7 @@ struct i915_refct_sgt *i915_rsgt_from_buddy_resource(struct ttm_resource *res,
> >  	GEM_BUG_ON(list_empty(blocks));
> >  	GEM_BUG_ON(!max_segment);
> >  
> > -	rsgt = kmalloc(sizeof(*rsgt), GFP_KERNEL);
> > +	rsgt = kmalloc(sizeof(*rsgt), GFP_KERNEL | __GFP_NOWARN);
> >  	if (!rsgt)
> >  		return ERR_PTR(-ENOMEM);
> >  
> > @@ -190,7 +190,7 @@ struct i915_refct_sgt *i915_rsgt_from_buddy_resource(struct ttm_resource *res,
> >  		return ERR_PTR(-E2BIG);
> >  	}
> >  
> > -	if (sg_alloc_table(st, PFN_UP(res->size), GFP_KERNEL)) {
> > +	if (sg_alloc_table(st, PFN_UP(res->size), GFP_KERNEL | __GFP_NOWARN)) {
> >  		i915_refct_sgt_put(rsgt);
> >  		return ERR_PTR(-ENOMEM);
> >  	}
> > -- 
> > 2.42.0
> >
> >       
>