linux-kernel - [RESEND PATCH] drm/amd/amdgpu: Replace kmap() with kmap_local

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Message-Id: <20221016174158.16638-1-fmdefrancesco@gmail.com>
Date:   Sun, 16 Oct 2022 19:41:58 +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>,
        Hawking Zhang <Hawking.Zhang@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Tao Zhou <tao.zhou1@....com>, Jack Xiao <Jack.Xiao@....com>,
        amd-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: [RESEND PATCH] drm/amd/amdgpu: Replace kmap() with kmap_local_page()

kmap() is being 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.

Since its use in amdgpu/amdgpu_ttm.c is safe, it should be preferred.

Therefore, replace kmap() with kmap_local_page() in amdgpu/amdgpu_ttm.c.

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

I'm resending because I suspect that this patch might have been lost. In
the meantime I added an "Acked-by" tag from Christian K.. Obviviously,
there are no further changes in the code.

 drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
index 3b4c19412625..c11657b5915f 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
@@ -2301,9 +2301,9 @@ static ssize_t amdgpu_iomem_read(struct file *f, char __user *buf,
 		if (p->mapping != adev->mman.bdev.dev_mapping)
 			return -EPERM;

-		ptr = kmap(p);
+		ptr = kmap_local_page(p);
 		r = copy_to_user(buf, ptr + off, bytes);
-		kunmap(p);
+		kunmap_local(ptr);
 		if (r)
 			return -EFAULT;

@@ -2352,9 +2352,9 @@ static ssize_t amdgpu_iomem_write(struct file *f, const char __user *buf,
 		if (p->mapping != adev->mman.bdev.dev_mapping)
 			return -EPERM;

-		ptr = kmap(p);
+		ptr = kmap_local_page(p);
 		r = copy_from_user(ptr + off, buf, bytes);
-		kunmap(p);
+		kunmap_local(ptr);
 		if (r)
 			return -EFAULT;

-- 
2.37.1