linux-kernel - Re: drm/i915: __GFP_RETRY_MAYFAIL allocations in stable kernels

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Date:   Fri, 18 Jun 2021 11:29:30 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Matthew Auld <matthew.william.auld@...il.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: drm/i915: __GFP_RETRY_MAYFAIL allocations in stable kernels

On (21/06/17 19:27), Daniel Vetter wrote:
> > 
> > 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.

Looks good.

> The commit that introduced a lot of this, including I915_GFP_ALLOW_FAIL
> seems to be
> 
> commit 1abb70f5955d1a9021f96359a2c6502ca569b68d
> Author: Chris Wilson <chris@...is-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.

Exactly, the commit we landed internally partially reverts 1abb70f5955
in 4.19 and 5.4 kernels. I don't mind I915_GFP_ALLOW_FAIL and so on, I
kept those bits, but we need reclaim. I can reproduce cases when order:0
allocation fails with
	__GFP_HIGHMEM|__GFP_RETRY_MAYFAIL
but succeeds with
	GFP_KERNEL|__GFP_HIGHMEM|__GFP_RETRY_MAYFAIL


ON a side note, I'm not very sure if __GFP_RETRY_MAYFAIL is actually
needed. Especially seeing it in syscalls is a bit uncommon:

  drm_ioctl()
   i915_gem_context_create_ioctl()
    i915_gem_create_context()
     i915_ppgtt_create()
      setup_scratch_page()           // __GFP_RETRY_MAYFAIL

But with GFP_KERNEL at least it tries to make some reclaim progress
between retries, so it seems to be good enough.