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SubjectRe: [PATCH v2 9/9] drm/i915: Use kmap_local_page() in gem/i915_gem_execbuffer.c
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On 31/03/2023 05:18, Ira Weiny wrote:
> 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], 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@intel.com
>>
>> 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@intel.com>
>> Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
>> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
>> ---
>> 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.

Reloc_cache_unmap is called from eb_relocate_entry.

The confusing part unmap appears first is just because reloc_cache is a
stateful setup. The previous mapping is kept around until reset (callers
moves to a different parent object), and unampped/remapped once moved to
a different page within that object.

However I am unsure if disabling pagefaulting is needed or not. Thomas,
Matt, being the last to touch this area, perhaps you could have a look?
Because I notice we have a fallback iomap path which still uses
io_mapping_map_atomic_wc. So if kmap_atomic to kmap_local conversion is
safe, does the iomap side also needs converting to
io_mapping_map_local_wc? Or they have separate requirements?

Regards,

Tvrtko

>
> 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
>>
>
>

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