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@xxxxxxxxx> > > > > 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 > 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@xxxxxxxxx > > > > Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx> > > Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx> > > Suggested-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx> > > Signed-off-by: Zhao Liu <zhao1.liu@xxxxxxxxx> > > --- > > 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 > >
