Re: [PATCH v2 9/9] drm/i915: Use kmap_local_page() in gem/i915_gem_execbuffer.c

[Zhao Liu]

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 Comment is a bit outdated since we don't use that global "struct mutex" any longer, but in any case, if there is a page fault on the mapping where we need to recurse into i915 again to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?

Thanks,
Zhao

> 
> Regards,
> 
> Tvrtko