linux-kernel - Re: [PATCH 1/7] drm/i915: Replace kmap() with kmap_local

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Date:   Mon, 13 Dec 2021 07:45:43 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Patrik Jakobsson <patrik.r.jakobsson@...il.com>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        linux-arm-msm@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org
Subject: Re: [PATCH 1/7] drm/i915: Replace kmap() with kmap_local_page()

On Mon, Dec 13, 2021 at 02:39:59PM +0200, Ville Syrjälä wrote:
> On Fri, Dec 10, 2021 at 03:23:58PM -0800, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > kmap() is being deprecated and these usages are all local to the thread
> > so there is no reason kmap_local_page() can't be used.
> > 
> > Replace kmap() calls with kmap_local_page().
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > ---
> >  drivers/gpu/drm/i915/gem/i915_gem_shmem.c          | 4 ++--
> >  drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c | 8 ++++----
> >  drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c       | 4 ++--
> >  drivers/gpu/drm/i915/gt/shmem_utils.c              | 4 ++--
> >  drivers/gpu/drm/i915/i915_gem.c                    | 8 ++++----
> >  drivers/gpu/drm/i915/i915_gpu_error.c              | 4 ++--
> >  6 files changed, 16 insertions(+), 16 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > index d77da59fae04..fa8b820e14aa 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > @@ -597,9 +597,9 @@ i915_gem_object_create_shmem_from_data(struct drm_i915_private *dev_priv,
> >  		if (err < 0)
> >  			goto fail;
> >  
> > -		vaddr = kmap(page);
> > +		vaddr = kmap_local_page(page);
> >  		memcpy(vaddr, data, len);
> > -		kunmap(page);
> > +		kunmap_local(vaddr);
> >  
> >  		err = pagecache_write_end(file, file->f_mapping,
> >  					  offset, len, len,
> > diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
> > index 6d30cdfa80f3..e59e1725e29d 100644
> > --- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
> > +++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c
> > @@ -144,7 +144,7 @@ static int check_partial_mapping(struct drm_i915_gem_object *obj,
> >  	intel_gt_flush_ggtt_writes(&to_i915(obj->base.dev)->gt);
> >  
> >  	p = i915_gem_object_get_page(obj, offset >> PAGE_SHIFT);
> > -	cpu = kmap(p) + offset_in_page(offset);
> > +	cpu = kmap_local_page(p) + offset_in_page(offset);
> 
> Does kunmap_local() do some magic to make it work even when you
> don't pass in the same value you got from kmap_local_page()?

Yes.  It sounds like a patch like this would be nice to clarify?

Ira

diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index 0a0b2b09b1b8..fb2d3e033c01 100644
--- a/include/linux/highmem-internal.h
+++ b/include/linux/highmem-internal.h
@@ -246,6 +246,17 @@ do {                                                               \
        __kunmap_atomic(__addr);                                \
 } while (0)
 
+/**
+ * kunmap_local - Unmap a page mapped via kmap_local_page().
+ * @__addr: An address within the page mapped
+ *
+ * __addr is often an address returned from kmap_local_page().  However,
+ * this address can be any address within the mapped page.  It does not need to
+ * be the exact address returned from kmap_local_page()
+ *
+ * Unmapping should be done in the reverse order of the mapping.  See
+ * kmap_local_page() for details.
+ */
 #define kunmap_local(__addr)                                   \
 do {                                                           \
        BUILD_BUG_ON(__same_type((__addr), struct page *));     \