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Date:   Fri, 14 Oct 2022 09:03:21 +0200
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Subject: Re: [PATCH] drm/radeon: Replace kmap() with kmap_local_page()
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To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Alex Deucher <alexander.deucher@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        Christian Brauner <brauner@kernel.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Jean Delvare <jdelvare@suse.com>,
        Guenter Roeck <linux@roeck-us.net>,
        Kees Cook <keescook@chromium.org>,
        amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org,
        linux-hwmon@vger.kernel.org, linux-hardening@vger.kernel.org
Cc:     "Venkataramanan, Anirudh" <anirudh.venkataramanan@intel.com>,
        Ira Weiny <ira.weiny@intel.com>
References: <20221013210714.16320-1-fmdefrancesco@gmail.com>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
In-Reply-To: <20221013210714.16320-1-fmdefrancesco@gmail.com>
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Am 13.10.22 um 23:07 schrieb Fabio M. De Francesco:
> 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@intel.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>

Reviewed-by: Christian König <christian.koenig@amd.com>

> ---
>   drivers/gpu/drm/radeon/radeon_ttm.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/radeon/radeon_ttm.c b/drivers/gpu/drm/radeon/radeon_ttm.c
> index d33fec488713..bdb4c0e0736b 100644
> --- a/drivers/gpu/drm/radeon/radeon_ttm.c
> +++ b/drivers/gpu/drm/radeon/radeon_ttm.c
> @@ -869,11 +869,11 @@ static ssize_t radeon_ttm_gtt_read(struct file *f, char __user *buf,
>   
>   		page = rdev->gart.pages[p];
>   		if (page) {
> -			ptr = kmap(page);
> +			ptr = kmap_local_page(page);
>   			ptr += off;
>   
>   			r = copy_to_user(buf, ptr, cur_size);
> -			kunmap(rdev->gart.pages[p]);
> +			kunmap_local(ptr);
>   		} else
>   			r = clear_user(buf, cur_size);
>