Re: [PATCH 1/9] drm/i915: Use kmap_local_page() in gem/i915_gem_object.c

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

On giovedì 3 novembre 2022 17:51:23 CET Ira Weiny wrote:
> On Sat, Oct 29, 2022 at 01:17:03PM +0200, Fabio M. De Francesco wrote:
> > On lunedì 17 ottobre 2022 11:37:17 CEST Zhao Liu wrote:
> > > From: Zhao Liu <zhao1.liu@intel.com>
> > > 
> > > The use of kmap_atomic() is being deprecated in favor of
> > > kmap_local_page()[1].
> > > 
> > > The main difference between atomic and local mappings is that local
> > > mappings doesn't disable page faults or preemption.
> > 
> > You are right about about page faults which are never disabled by
> > kmap_local_page(). However kmap_atomic might not disable preemption. It
> > depends on CONFIG_PREEMPT_RT.
> > 
> > Please refer to how kmap_atomic_prot() works (this function is called by
> > kmap_atomic() when kernels have HIGHMEM enabled).
> > 
> > > There're 2 reasons why i915_gem_object_read_from_page_kmap() doesn't
> > > need to disable pagefaults and preemption for mapping:
> > > 
> > > 1. The flush operation is safe for CPU hotplug when preemption is not
> > > disabled.
> > 
> > I'm confused here. Why are you talking about CPU hotplug?
> 
> I agree with Fabio here.  I'm not making the connection between cpu hotplug 
and
> this code path.
> 
> Ira

@Zhao,

I'd like to add that I was about to put my reviewed-by tag. The other things I 
objected are minor nits. Please just clarify this connection.

Your code is good and deserves to be applied.

Fabio

> 
> > In any case, developers should never rely on implicit calls of
> > preempt_disable() for the reasons said above. Therefore, flush operations
> > should be allowed regardless that kmap_atomic() potential side effect.
> > 
> > > In drm/i915/gem/i915_gem_object.c, the function
> > > i915_gem_object_read_from_page_kmap() calls drm_clflush_virt_range()
> > 
> > If I recall correctly, drm_clflush_virt_range() can always be called with 
page
> > faults and preemption enabled. If so, this is enough to say that the
> > conversion is safe.
> > 
> > Is this code explicitly related to flushing the cache lines before 
removing /
> > adding CPUs? If I recall correctly, there are several other reasons behind 
the
> > need to issue cache lines flushes. Am I wrong about this?
> > 
> > Can you please say more about what I'm missing here?
> > 
> > > to
> > > use CLFLUSHOPT or WBINVD to flush. Since CLFLUSHOPT is global on x86
> > > and WBINVD is called on each cpu in drm_clflush_virt_range(), the flush
> > > operation is global and any issue with cpu's being added or removed
> > > can be handled safely.
> > 
> > Again your main concern is about CPU hotplug.
> > 
> > Even if I'm missing something, do we really need all these details about 
the
> > inner workings of drm_clflush_virt_range()?
> > 
> > I'm not an expert, so may be that I'm wrong about all I wrote above.
> > 
> > Therefore, can you please elaborate a little more for readers with very 
little
> > knowledge of these kinds of things (like me and perhaps others)?
> > 
> > > 2. Any context switch caused by preemption or sleep (pagefault may
> > > cause sleep) doesn't affect the validity of local mapping.
> > 
> > I'd replace "preemption or sleep" with "preemption and page faults" since
> > yourself then added that page faults lead to tasks being put to sleep.
> > 
> > > Therefore, i915_gem_object_read_from_page_kmap() is a function where
> > > the use of kmap_local_page() in place of kmap_atomic() is correctly
> > > suited.
> > > 
> > > Convert the calls of kmap_atomic() / kunmap_atomic() to
> > > kmap_local_page() / kunmap_local().
> > > 
> > > And remove the redundant variable that stores the address of the mapped
> > > page since kunmap_local() can accept any pointer within the page.
> > > 
> > > [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com
> > > 
> > > Suggested-by: Dave Hansen <dave.hansen@intel.com>
> > > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > > Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > > ---
> > > 
> > > Suggested by credits:
> > >   Dave: Referred to his explanation about cache flush.
> > >   Ira: Referred to his task document, review comments and explanation 
about
> > >   
> > >        cache flush.
> > >   
> > >   Fabio: Referred to his boiler plate commit message.
> > > 
> > > ---
> > > 
> > >  drivers/gpu/drm/i915/gem/i915_gem_object.c | 8 +++-----
> > >  1 file changed, 3 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c
> > > b/drivers/gpu/drm/i915/gem/i915_gem_object.c index
> > 
> > 369006c5317f..a0072abed75e 100644
> > 
> > > --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c
> > > +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c
> > > @@ -413,17 +413,15 @@ void 
__i915_gem_object_invalidate_frontbuffer(struct
> > > drm_i915_gem_object *obj, static void
> > > 
> > >  i915_gem_object_read_from_page_kmap(struct drm_i915_gem_object *obj, 
u64
> > 
> > offset, void
> > 
> > > *dst, int size) {
> > > -	void *src_map;
> > > 
> > >  	void *src_ptr;
> > > 
> > > -	src_map = kmap_atomic(i915_gem_object_get_page(obj, offset >>
> > 
> > PAGE_SHIFT));
> > 
> > > -
> > > -	src_ptr = src_map + offset_in_page(offset);
> > > +	src_ptr = kmap_local_page(i915_gem_object_get_page(obj, offset >>
> > 
> > PAGE_SHIFT))
> > 
> > > +	          + offset_in_page(offset);
> > > 
> > >  	if (!(obj->cache_coherent & I915_BO_CACHE_COHERENT_FOR_READ))
> > >  	
> > >  		drm_clflush_virt_range(src_ptr, size);
> > >  	
> > >  	memcpy(dst, src_ptr, size);
> > > 
> > > -	kunmap_atomic(src_map);
> > > +	kunmap_local(src_ptr);
> > > 
> > >  }
> > >  
> > >  static void
> > 
> > The changes look good, but I'd like to better understand the commit 
message.
> > 
> > Thanks,
> > 
> > Fabio