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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Matthew Auld <matthew.auld@intel.com>,
        Thomas =?ISO-8859-1?Q?Hellstr=F6m?= 
        <thomas.hellstrom@linux.intel.com>,
        Nirmoy Das <nirmoy.das@intel.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Christian =?ISO-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, Zhao Liu <zhao1.liu@linux.intel.com>
Cc:     Ira Weiny <ira.weiny@intel.com>,
        Zhenyu Wang <zhenyu.z.wang@intel.com>,
        Zhao Liu <zhao1.liu@intel.com>
Subject: Re: [PATCH 3/9] drm/i915: Use kmap_local_page() in gem/i915_gem_shmem.c
Date:   Thu, 03 Nov 2022 23:22:32 +0100
Message-ID: <1833466.tdWV9SEqCh@suse>
In-Reply-To: <20221017093726.2070674-4-zhao1.liu@linux.intel.com>
References: <20221017093726.2070674-1-zhao1.liu@linux.intel.com> <20221017093726.2070674-4-zhao1.liu@linux.intel.com>
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On luned=EC 17 ottobre 2022 11:37:19 CET Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@intel.com>
>=20
> The use of kmap_atomic() is being deprecated in favor of
> kmap_local_page()[1].
>=20
> The main difference between atomic and local mappings is that local
> mappings doesn't disable page faults or preemption.
>=20
> In drm/i915/gem/i915_gem_shmem.c, the function shmem_pwrite() need to
> disable pagefault to eliminate the potential recursion fault[2]. But
> here __copy_from_user_inatomic() doesn't need to disable preemption and
> local mapping is valid for sched in/out.
> So it can use kmap_local_page() / kunmap_local() with
> pagefault_disable() / pagefault_enable() to replace atomic mapping.
>=20
> [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.=
com
> [2]: https://patchwork.freedesktop.org/patch/295840/
>=20
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> ---
> Suggested by credits:
>   Ira: Referred to his suggestions about keeping pagefault_disable().
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)

The commit message explains clearly and the changes to the code look good. =
The=20
necessity to reuse pagefault_disable() / pagefault_enable() from the main=20
kmap_atomic() side effect is a nice catch.

Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

Thanks!

> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c index f42ca1179f37..e279a3e30=
c02=20
100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> @@ -472,11 +472,13 @@ shmem_pwrite(struct drm_i915_gem_object *obj,
>  		if (err < 0)
>  			return err;
>=20
> -		vaddr =3D kmap_atomic(page);
> +		vaddr =3D kmap_local_page(page);
> +		pagefault_disable();
>  		unwritten =3D __copy_from_user_inatomic(vaddr + pg,
>  						      user_data,
>  						      len);
> -		kunmap_atomic(vaddr);
> +		pagefault_enable();
> +		kunmap_local(vaddr);
>=20
>  		err =3D aops->write_end(obj->base.filp, mapping, offset,=20
len,
>  				      len - unwritten, page, data);