Messages in this thread | | | Date | Mon, 17 Apr 2023 12:24:45 +0100 | Subject | Re: [PATCH v2 9/9] drm/i915: Use kmap_local_page() in gem/i915_gem_execbuffer.c | From | Tvrtko Ursulin <> |
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On 14/04/2023 11:45, Zhao Liu wrote: > Hi Tvrtko, > > On Wed, Apr 12, 2023 at 04:45:13PM +0100, Tvrtko Ursulin wrote: > > [snip] > >>> >>> [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 Comment is a bit outdated since we don't use that global "struct mutex" any longer, but in any case, if there is a page fault on the mapping where we need to recurse into i915 again to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. > > Thanks for your explanation! > >> >> Patch to convert the io_mapping_map_atomic_wc can indeed come later. > > Okay, I will also look at this. > >> >> 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? > > Yeah, it's ok for me. But could I ask, did I miss the 6.4 merge time?
Yes, but just because we failed to review and merge in time, not because you did not provide patches in time.
Regards,
Tvrtko
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