On 31/03/2023 16:32, Fabio M. De Francesco wrote:
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?
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.
Patch to convert the io_mapping_map_atomic_wc can indeed come later.
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?
Regards,
Tvrtko
