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Date:   Thu, 30 Mar 2023 20:04:39 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     Zhao Liu <zhao1.liu@linux.intel.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
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        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>,
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CC:     Ira Weiny <ira.weiny@intel.com>,
        "Fabio M . De Francesco" <fmdefrancesco@gmail.com>,
        Zhenyu Wang <zhenyu.z.wang@intel.com>,
        Zhao Liu <zhao1.liu@intel.com>,
        Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH v2 4/9] drm/i915: Use kmap_local_page() in
 gem/selftests/huge_pages.c
Message-ID: <64264dc74c39_375f7e2948b@iweiny-mobl.notmuch>
References: <20230329073220.3982460-1-zhao1.liu@linux.intel.com>
 <20230329073220.3982460-5-zhao1.liu@linux.intel.com>
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Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@intel.com>
> 
> The use of kmap_atomic() is being deprecated in favor of
> kmap_local_page()[1], and this patch converts the call from
> kmap_atomic() to kmap_local_page().
> 
> The main difference between atomic and local mappings is that local
> mappings doesn't disable page faults or preemption (the preemption is
> disabled for !PREEMPT_RT case, otherwise it only disables migration).
> 
> With kmap_local_page(), we can avoid the often unwanted side effect of
> unnecessary page faults or preemption disables.
> 
> In drm/i915/gem/selftests/huge_pages.c, function __cpu_check_shmem()
> mainly uses mapping to flush cache and check the value. There're
> 2 reasons why __cpu_check_shmem() doesn't need to disable pagefaults
> and preemption for mapping:
> 
> 1. The flush operation is safe. Function __cpu_check_shmem() calls
> drm_clflush_virt_range() to use CLFLUSHOPT or WBINVD to flush. Since
> CLFLUSHOPT is global on x86 and WBINVD is called on each cpu in
> drm_clflush_virt_range(), the flush operation is global.
> 
> 2. Any context switch caused by preemption or page faults (page fault
> may cause sleep) doesn't affect the validity of local mapping.
> 
> Therefore, __cpu_check_shmem() is a function where the use of
> kmap_local_page() in place of kmap_atomic() is correctly suited.
> 
> Convert the calls of kmap_atomic() / kunmap_atomic() to
> kmap_local_page() / kunmap_local().
> 
> [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com
> 
> v2:
> * Dropped hot plug related description since it has nothing to do with
>   kmap_local_page().
> * No code change since v1, and added description of the motivation of
>   using kmap_local_page().
> 
> Suggested-by: Dave Hansen <dave.hansen@intel.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>

Reviewed-by: Ira Weiny <ira.weiny@intel.com>