On Mon, Apr 17, 2023 at 12:24:45PM +0100, Tvrtko Ursulin wrote:
>
> 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 Comment is a bit outdated since we don't use that global "struct mutex"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.
It is worth mentioning that under drm we close the merge window earlier.
Around -rc5.
So, Linus' merge window for 6.4 didn't happen yet. But our drm-next that
is going to be sent there is already closed.
>
> Regards,
>
> Tvrtko
>
