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

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Message-Id: <20221013210714.16320-1-fmdefrancesco@gmail.com>
Date:   Thu, 13 Oct 2022 23:07:14 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     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>,
        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:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH] drm/radeon: Replace kmap() with kmap_local_page()

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>
---
 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);
 
-- 
2.37.3