linux-kernel - [PATCH v2 1/3] drm/i915: Replace kmap() with kmap_local

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Date:   Mon, 17 Apr 2023 12:38:52 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     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>,
        Matt Roper <matthew.d.roper@...el.com>,
        John Harrison <John.C.Harrison@...el.com>,
        Daniele Ceraolo Spurio <daniele.ceraolospurio@...el.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH v2 1/3] drm/i915: Replace kmap() with kmap_local_page()

kmap() has been deprecated in favor of kmap_local_page().

There are two main problems with kmap(): (1) It comes with an overhead as
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 are still valid.

Obviously, thread locality implies that the kernel voirtual addresses
are valid only in the context of the callers. kmap_local_page() use in
i915_gem.c doesn't break the above-mentioned constraint, so it should be
preferred to kmap().

Therefore, replace kmap() with kmap_local_page() in i915_gem.c

Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---
 drivers/gpu/drm/i915/i915_gem.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 35950fa91406..c35248555e42 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -212,14 +212,14 @@ shmem_pread(struct page *page, int offset, int len, char __user *user_data,
 	char *vaddr;
 	int ret;

-	vaddr = kmap(page);
+	vaddr = kmap_local_page(page);

 	if (needs_clflush)
 		drm_clflush_virt_range(vaddr + offset, len);

 	ret = __copy_to_user(user_data, vaddr + offset, len);

-	kunmap(page);
+	kunmap_local(vaddr);

 	return ret ? -EFAULT : 0;
 }
@@ -643,7 +643,7 @@ shmem_pwrite(struct page *page, int offset, int len, char __user *user_data,
 	char *vaddr;
 	int ret;

-	vaddr = kmap(page);
+	vaddr = kmap_local_page(page);

 	if (needs_clflush_before)
 		drm_clflush_virt_range(vaddr + offset, len);
@@ -652,7 +652,7 @@ shmem_pwrite(struct page *page, int offset, int len, char __user *user_data,
 	if (!ret && needs_clflush_after)
 		drm_clflush_virt_range(vaddr + offset, len);

-	kunmap(page);
+	kunmap_local(vaddr);

 	return ret ? -EFAULT : 0;
 }
--
2.40.0