Re: drm/i915: __GFP_RETRY_MAYFAIL allocations in stable kernels

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jun 14, 2021 at 09:45:37PM +0900, Sergey Senozhatsky wrote:
> Hi,
> 
> We are observing some user-space crashes (sigabort, segfaults etc.)
> under moderate memory pressure (pretty far from severe pressure) which
> have one thing in common - restrictive GFP mask in setup_scratch_page().
> 
> For instance, (stable 4.19) drivers/gpu/drm/i915/i915_gem_gtt.c
> 
> (trimmed down version)
> 
> static int gen8_init_scratch(struct i915_address_space *vm)
> {
>         setup_scratch_page(vm, __GFP_HIGHMEM);
> 
>         vm->scratch_pt = alloc_pt(vm);
>         vm->scratch_pd = alloc_pd(vm);
>         if (use_4lvl(vm)) {
>                 vm->scratch_pdp = alloc_pdp(vm);
>         }
> }
> 
> gen8_init_scratch() function puts a rather inconsistent restrictions on mm.
> 
> Looking at it line by line:
> 
> setup_scratch_page() uses very restrictive gfp mask:
> 	__GFP_HIGHMEM | __GFP_ZERO | __GFP_RETRY_MAYFAIL
> 
> it doesn't try to reclaim anything and fails almost immediately.
> 
> alloc_pt() - uses more permissive gfp mask:
> 	GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN
> 
> alloc_pd() - likewise:
> 	GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN
> 
> alloc_pdp() - very permissive gfp mask:
> 	GFP_KERNEL
> 
> 
> So can all allocations in gen8_init_scratch() use
> 	GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN

Yeah that looks all fairly broken tbh. The only thing I didn't know was
that GFP_DMA32 wasn't a full gfp mask with reclaim bits set as needed. I
guess it would be clearer if we use GFP_KERNEL | __GFP_DMA32 for these.

The commit that introduced a lot of this, including I915_GFP_ALLOW_FAIL
seems to be

commit 1abb70f5955d1a9021f96359a2c6502ca569b68d
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue May 22 09:36:43 2018 +0100

    drm/i915/gtt: Allow pagedirectory allocations to fail

which used a selftest as justification, not real world workloads, so looks
rather dubious.

Adding Matt Auld to this thread, maybe he has ideas.

Thanks, Daniel

> ?
> 
> E.g.
> 
> ---
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index a12430187108..e862680b9c93 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -792,7 +792,7 @@ alloc_pdp(struct i915_address_space *vm)
>  
>         GEM_BUG_ON(!use_4lvl(vm));
>  
> -       pdp = kzalloc(sizeof(*pdp), GFP_KERNEL);
> +       pdp = kzalloc(sizeof(*pdp), I915_GFP_ALLOW_FAIL);
>         if (!pdp)
>                 return ERR_PTR(-ENOMEM);
>  
> @@ -1262,7 +1262,7 @@ static int gen8_init_scratch(struct i915_address_space *vm)
>  {
>         int ret;
>  
> -       ret = setup_scratch_page(vm, __GFP_HIGHMEM);
> +       ret = setup_scratch_page(vm, GFP_KERNEL | __GFP_HIGHMEM);
>         if (ret)
>                 return ret;
>  
> @@ -1972,7 +1972,7 @@ static int gen6_ppgtt_init_scratch(struct gen6_hw_ppgtt *ppgtt)
>         u32 pde;
>         int ret;
>  
> -       ret = setup_scratch_page(vm, __GFP_HIGHMEM);
> +       ret = setup_scratch_page(vm, GFP_KERNEL | __GFP_HIGHMEM);
>         if (ret)
>                 return ret;
>  
> @@ -3078,7 +3078,7 @@ static int ggtt_probe_common(struct i915_ggtt *ggtt, u64 size)
>                 return -ENOMEM;
>         }
>  
> -       ret = setup_scratch_page(&ggtt->vm, GFP_DMA32);
> +       ret = setup_scratch_page(&ggtt->vm, GFP_KERNEL | GFP_DMA32);
>         if (ret) {
>                 DRM_ERROR("Scratch setup failed\n");
>                 /* iounmap will also get called at remove, but meh */
> ---
> 
> 
> 
> It's quite similar on stable 5.4 - setup_scratch_page() uses restrictive
> gfp mask again.
> 
> ---
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index f614646ed3f9..99d78b1052df 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -1378,7 +1378,7 @@ static int gen8_init_scratch(struct i915_address_space *vm)
>                 return 0;
>         }
>  
> -       ret = setup_scratch_page(vm, __GFP_HIGHMEM);
> +       ret = setup_scratch_page(vm, GFP_KERNEL | __GFP_HIGHMEM);
>         if (ret)
>                 return ret;
>  
> @@ -1753,7 +1753,7 @@ static int gen6_ppgtt_init_scratch(struct gen6_ppgtt *ppgtt)
>         struct i915_page_directory * const pd = ppgtt->base.pd;
>         int ret;
>  
> -       ret = setup_scratch_page(vm, __GFP_HIGHMEM);
> +       ret = setup_scratch_page(vm, GFP_KERNEL | __GFP_HIGHMEM);
>         if (ret)
>                 return ret;
>  
> @@ -2860,7 +2860,7 @@ static int ggtt_probe_common(struct i915_ggtt *ggtt, u64 size)
>                 return -ENOMEM;
>         }
>  
> -       ret = setup_scratch_page(&ggtt->vm, GFP_DMA32);
> +       ret = setup_scratch_page(&ggtt->vm, GFP_KERNEL | GFP_DMA32);
>         if (ret) {
>                 DRM_ERROR("Scratch setup failed\n");
>                 /* iounmap will also get called at remove, but meh */
> ---

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch