Re: [PATCH v2 9/9] drm/i915: Use kmap_local_page() in gem/i915_gem_execbuffer.c

[Ira Weiny]

Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@xxxxxxxxx>
> 
> The use of kmap_atomic() is being deprecated in favor of
> kmap_local_page()[1], and this patch converts the calls from
> kmap_atomic() to kmap_local_page().
> 
> The main difference between atomic and local mappings is that local
> mappings doesn't disable page faults or preemption (the preemption is
> disabled for !PREEMPT_RT case, otherwise it only disables migration).
> 
> With kmap_local_page(), we can avoid the often unwanted side effect of
> unnecessary page faults and preemption disables.
> 
> In i915_gem_execbuffer.c, eb->reloc_cache.vaddr is mapped by
> kmap_atomic() in eb_relocate_entry(), and is unmapped by
> kunmap_atomic() in reloc_cache_reset().

First off thanks for the series and sticking with this.  That said this
patch kind of threw me for a loop because tracing the map/unmap calls did
not make sense to me.  See below.

> 
> And this mapping/unmapping occurs in two places: one is in
> eb_relocate_vma(), and another is in eb_relocate_vma_slow().
> 
> The function eb_relocate_vma() or eb_relocate_vma_slow() doesn't
> need to disable pagefaults and preemption during the above mapping/
> unmapping.
> 
> So it can simply use kmap_local_page() / kunmap_local() that can
> instead do the mapping / unmapping regardless of the context.
> 
> Convert the calls of kmap_atomic() / kunmap_atomic() to
> kmap_local_page() / kunmap_local().
> 
> [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@xxxxxxxxx
> 
> v2: No code change since v1. Added description of the motivation of
>     using kmap_local_page() and "Suggested-by" tag of Fabio.
> 
> 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:
>   Ira: Referred to his task document, review comments.
>   Fabio: Referred to his boiler plate commit message and his description
>          about why kmap_local_page() should be preferred.
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> index 9dce2957b4e5..805565edd148 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> @@ -1151,7 +1151,7 @@ static void reloc_cache_unmap(struct reloc_cache *cache)
>  
>  	vaddr = unmask_page(cache->vaddr);
>  	if (cache->vaddr & KMAP)
> -		kunmap_atomic(vaddr);
> +		kunmap_local(vaddr);

In the cover letter you don't mention this unmap path.  Rather you mention
only reloc_cache_reset().

After digging into this and considering these are kmap_atomic() calls I
_think_ what you have is ok.  But I think I'd like to see the call paths
documented a bit more clearly.  Or perhaps cleaned up a lot.

For example I see the following call possibility from a user ioctl.  In
this trace I see 2 examples where something is unmapped first.  I don't
understand why that is required?  I would assume reloc_cache_unmap() and
reloc_kmap() are helpers called from somewhere else requiring a remapping
of the cache but I don't see it.

i915_gem_execbuffer2_ioctl()
eb_relocate_parse()
eb_relocate_parse_slow()
eb_relocate_vma_slow()
	eb_relocate_entry()
		reloc_cache_unmap()
			kunmap_atomic()  <=== HERE!
		reloc_cache_remap()
			kmap_atomic()
		relocate_entry()
			reloc_vaddr()
				reloc_kmap()
					kunmap_atomic() <== HERE!
					kmap_atomic()

	reloc_cache_reset()
		kunmap_atomic()

Could these mappings be cleaned up a lot more?  Perhaps by removing some
of the helper functions which AFAICT are left over from older versions of
the code?

Also as an aside I think it is really bad that eb_relocate_entry() returns
negative errors in a u64.  Better to get the types right IMO.

Thanks for the series!
Ira

>  	else
>  		io_mapping_unmap_atomic((void __iomem *)vaddr);
>  }
> @@ -1167,7 +1167,7 @@ static void reloc_cache_remap(struct reloc_cache *cache,
>  	if (cache->vaddr & KMAP) {
>  		struct page *page = i915_gem_object_get_page(obj, cache->page);
>  
> -		vaddr = kmap_atomic(page);
> +		vaddr = kmap_local_page(page);
>  		cache->vaddr = unmask_flags(cache->vaddr) |
>  			(unsigned long)vaddr;
>  	} else {
> @@ -1197,7 +1197,7 @@ static void reloc_cache_reset(struct reloc_cache *cache, struct i915_execbuffer
>  		if (cache->vaddr & CLFLUSH_AFTER)
>  			mb();
>  
> -		kunmap_atomic(vaddr);
> +		kunmap_local(vaddr);
>  		i915_gem_object_finish_access(obj);
>  	} else {
>  		struct i915_ggtt *ggtt = cache_to_ggtt(cache);
> @@ -1229,7 +1229,7 @@ static void *reloc_kmap(struct drm_i915_gem_object *obj,
>  	struct page *page;
>  
>  	if (cache->vaddr) {
> -		kunmap_atomic(unmask_page(cache->vaddr));
> +		kunmap_local(unmask_page(cache->vaddr));
>  	} else {
>  		unsigned int flushes;
>  		int err;
> @@ -1251,7 +1251,7 @@ static void *reloc_kmap(struct drm_i915_gem_object *obj,
>  	if (!obj->mm.dirty)
>  		set_page_dirty(page);
>  
> -	vaddr = kmap_atomic(page);
> +	vaddr = kmap_local_page(page);
>  	cache->vaddr = unmask_flags(cache->vaddr) | (unsigned long)vaddr;
>  	cache->page = pageno;
>  
> -- 
> 2.34.1
>