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

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Message-ID: <fb0b7389-7121-04f8-176d-1ababe0ad8f2@amd.com>
Date:   Fri, 14 Oct 2022 09:03:21 +0200
From:   Christian König <christian.koenig@....com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Alex Deucher <alexander.deucher@....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>,
        Kees Cook <keescook@...omium.org>,
        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
Cc:     "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] drm/radeon: Replace kmap() with kmap_local_page()

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@...el.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Reviewed-by: Christian König <christian.koenig@....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);
>