linux-hardening - Re: [PATCH] drm/radeon: Replace kmap() with kmap_local

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Date:   Thu, 13 Oct 2022 19:41:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Christian Brauner <brauner@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        linux-hwmon@...r.kernel.org, linux-hardening@...r.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] drm/radeon: Replace kmap() with kmap_local_page()

On Thu, Oct 13, 2022 at 11:07:14PM +0200, Fabio M. De Francesco wrote:
> The use of kmap() is being deprecated in favor of kmap_local_page().
> 
> There are two main problems with kmap(): (1) It comes with an overhead as
> the mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap’s pool wraps and it might block when the mapping space is fully
> utilized until a slot becomes available.
> 
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and still valid.
> 
> Therefore, replace kmap() with kmap_local_page() in radeon_ttm_gtt_read().
> 
> Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook