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Comment is a bit outdated since we don't use that global "struct mutex" any longer, but in any case, if there is a page fault on the mapping where we need to recurse into i915 again to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