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@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(-) > >> [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? Thanks, Fabio > Regards, > > Tvrtko
