linux-kernel - Re: [PATCH v2 5/9] drm/i915: Use kmap_local_page() in gem/selftests/i915_gem

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <64264e64af269_375f7e2948f@iweiny-mobl.notmuch>
Date:   Thu, 30 Mar 2023 20:07:16 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Zhao Liu <zhao1.liu@...ux.intel.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Matthew Auld <matthew.auld@...el.com>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>,
        Nirmoy Das <nirmoy.das@...el.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Christian König <christian.koenig@....com>,
        <intel-gfx@...ts.freedesktop.org>,
        <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
CC:     Ira Weiny <ira.weiny@...el.com>,
        "Fabio M . De Francesco" <fmdefrancesco@...il.com>,
        Zhenyu Wang <zhenyu.z.wang@...el.com>,
        Zhao Liu <zhao1.liu@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v2 5/9] drm/i915: Use kmap_local_page() in
 gem/selftests/i915_gem_coherency.c

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], 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/i915_gem_coherency.c, functions cpu_set()
> and cpu_get() mainly uses mapping to flush cache and assign the value.
> There're 2 reasons why cpu_set() and cpu_get() don't need to disable
> pagefaults and preemption for mapping:
> 
> 1. The flush operation is safe. cpu_set() and cpu_get() call
> 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_set() and cpu_get() are functions 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@...el.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>

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