Re: [PATCH v2 9/9] drm/i915: Use kmap_local_page() in gem/i915_gem_execbuffer.c
From: Tvrtko Ursulin
Date: Fri Mar 31 2023 - 07:31:53 EST
On 31/03/2023 05:18, Ira Weiny wrote:
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.
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