linux-kernel - Re: [PATCH 1/9] drm/i915: Use kmap_local_page() in gem/i915_gem

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Date:   Fri, 4 Nov 2022 19:44:05 +0800
From:   Zhao Liu <zhao1.liu@...ux.intel.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Zhenyu Wang <zhenyu.z.wang@...el.com>,
        Zhao Liu <zhao1.liu@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH 1/9] drm/i915: Use kmap_local_page() in
 gem/i915_gem_object.c

On Thu, Nov 03, 2022 at 08:22:04PM +0100, Fabio M. De Francesco wrote:
> Date: Thu, 03 Nov 2022 20:22:04 +0100
> From: "Fabio M. De Francesco" <fmdefrancesco@...il.com>
> Subject: Re: [PATCH 1/9] drm/i915: Use kmap_local_page() in
>  gem/i915_gem_object.c
> 
> On gioved? 3 novembre 2022 17:51:23 CET Ira Weiny wrote:
> > On Sat, Oct 29, 2022 at 01:17:03PM +0200, Fabio M. De Francesco wrote:
> > > On luned? 17 ottobre 2022 11:37:17 CEST Zhao Liu wrote:
> > > > From: Zhao Liu <zhao1.liu@...el.com>
> > > > 
> > > > The use of kmap_atomic() is being deprecated in favor of
> > > > kmap_local_page()[1].
> > > > 
> > > > The main difference between atomic and local mappings is that local
> > > > mappings doesn't disable page faults or preemption.
> > > 
> > > You are right about about page faults which are never disabled by
> > > kmap_local_page(). However kmap_atomic might not disable preemption. It
> > > depends on CONFIG_PREEMPT_RT.
> > > 
> > > Please refer to how kmap_atomic_prot() works (this function is called by
> > > kmap_atomic() when kernels have HIGHMEM enabled).
> > > 
> > > > There're 2 reasons why i915_gem_object_read_from_page_kmap() doesn't
> > > > need to disable pagefaults and preemption for mapping:
> > > > 
> > > > 1. The flush operation is safe for CPU hotplug when preemption is not
> > > > disabled.
> > > 
> > > I'm confused here. Why are you talking about CPU hotplug?
> > 
> > I agree with Fabio here.  I'm not making the connection between cpu hotplug 
> and
> > this code path.
> > 
> > Ira
> 
> @Zhao,
> 
> I'd like to add that I was about to put my reviewed-by tag. The other things I 
> objected are minor nits. Please just clarify this connection.

Thanks Fabio for your comments! Sorry I missed the mails that day. This connection
is my misunderstanding. Other thoughts please refer to my reply to your first email
in this thread.

Thanks,
Zhao