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

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Date:   Mon, 17 Oct 2022 12:53:24 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     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, Ira Weiny <ira.weiny@...el.com>
Subject: Re: [RESEND PATCH] drm/amd/amdgpu: Replace kmap() with kmap_local_page()

Applied.  Thanks!

On Sun, Oct 16, 2022 at 1:42 PM Fabio M. De Francesco
<fmdefrancesco@...il.com> wrote:
>
> 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
>