Messages in this thread | | | Date | Wed, 12 Apr 2023 16:45:13 +0100 | Subject | Re: [PATCH v2 9/9] drm/i915: Use kmap_local_page() in gem/i915_gem_execbuffer.c | From | Tvrtko Ursulin <> |
| |
On 31/03/2023 16:32, Fabio M. De Francesco wrote: > On venerdì 31 marzo 2023 13:30:20 CEST Tvrtko Ursulin wrote: >> 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(-) >>>> > > [snip] > >> 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? > > AFAIK, the requirements for io_mapping_map_local_wc() are the same as for > kmap_local_page(): the kernel virtual address is _only_ valid in the caller > context, and map/unmap nesting must be done in stack-based ordering (LIFO). > > I think a follow up patch could safely switch to io_mapping_map_local_wc() / > io_mapping_unmap_local_wc since the address is local to context. > > However, not being an expert, reading your note now I suspect that I'm missing > something. Can I ask why you think that page-faults disabling might be > necessary?
I am not saying it is, was just unsure and wanted some people who worked on this code most recently to take a look and confirm.
I guess it will work since the copying is done like this anyway:
/* * This is the fast path and we cannot handle a pagefault * whilst holding the struct mutex lest the user pass in the * relocations contained within a mmaped bo. For in such a case * we, the page fault handler would call i915_gem_fault() and * we would try to acquire the struct mutex again. Obviously * this is bad and so lockdep complains vehemently. */ pagefault_disable(); copied = __copy_from_user_inatomic(r, urelocs, count * sizeof(r[0])); pagefault_enable(); if (unlikely(copied)) { remain = -EFAULT; goto out; }
Comment is a bit outdated since we don't use that global "struct mutex" any longer, but in any case, if there is a page fault on the mapping where we need to recurse into i915 again to satisfy if, we seem to have code already to handle it. So kmap_local conversion I *think* can't regress anything.
Patch to convert the io_mapping_map_atomic_wc can indeed come later.
In terms of logistics - if we landed this series to out branch it would be queued only for 6.5. Would that work for you?
Regards,
Tvrtko
| |