Re: [PATCH v2] drm/i915: Replace kmap() with kmap_local_page()

[Thomas Hellström (Intel)]

On 6/20/23 20:07, Sumitra Sharma wrote:
> On Tue, Jun 20, 2023 at 06:23:38AM -0700, Ira Weiny wrote:
> > Sumitra Sharma wrote:
> > > On Sun, Jun 18, 2023 at 11:11:08AM -0700, Ira Weiny wrote:
> > > > Sumitra Sharma wrote:
> > > > > kmap() has been deprecated in favor of the kmap_local_page()
> > > > > due to high cost, restricted mapping space, the overhead of a
> > > > > global lock for synchronization, and making the process sleep
> > > > > in the absence of free slots.
> > > > >
> > > > > kmap_local_page() is faster than kmap() and offers thread-local
> > > > > and CPU-local mappings, take pagefaults in a local kmap region
> > > > > and preserves preemption by saving the mappings of outgoing tasks
> > > > > and restoring those of the incoming one during a context switch.
> > > > >
> > > > > The mapping is kept thread local in the function
> > > > > “i915_vma_coredump_create” in i915_gpu_error.c
> > > > >
> > > > > Therefore, replace kmap() with kmap_local_page().
> > > > >
> > > > > Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > > NIT: No need for the line break between Suggested-by and your signed off line.
> > > Hi Ira,
> > >
> > > What does NIT stand for?
> > Shorthand for 'nitpicking'.
> >
> > "giving too much attention to details that are not important, especially
> > as a way of criticizing: "
> >
> > 	- https://dictionary.cambridge.org/dictionary/english/nitpicking
> >
> > Via email this is a way for authors of an email to indicate something is
> > technically wrong but while nicely acknowledging that it is not very
> > significant and could be seen as overly critical.
> >
> > For this particular comment I'm showing something to pay attention to next
> > time but that was not a big deal this time around.
> Hi Ira,
>
> Thank for your explanation on NIT.
>
>
> > > Thank you. I will take care about the line breaks.
> > >
> > > > > Signed-off-by: Sumitra Sharma <sumitraartsy@xxxxxxxxx>
> > > > > ---
> > > > >
> > > > > Changes in v2:
> > > > > 	- Replace kmap() with kmap_local_page().
> > > > Generally it is customary to attribute a change like this to those who
> > > > suggested it in a V1 review.
> > > >
> > > > For example:
> > > >
> > > >   	- Tvrtko/Thomas: Use kmap_local_page() instead of page_address()
> > > >
> > > > Also I don't see Thomas on the new email list.  Since he took the time to
> > > > review V1 he might want to check this version out.  I've added him to the
> > > > 'To:' list.
> > > >
> > > > Also a link to V1 is nice.  B4 formats it like this:
> > > >
> > > > - Link to v1: https://lore.kernel.org/all/20230614123556.GA381200@xxxxxxxxxxx/
> > > >
> > > > All that said the code looks good to me.  So with the above changes.
> > > >
> > > > Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > I have noted down the points mentioned above. Thank you again.
> > >
> > > I am not supposed to create another version of this patch for
> > > adding the above mentions, as you and Thomas both gave this patch
> > > a reviewed-by tag. Right?
> > Based on this response[*] from Tvrtko I think this version can move
> > through without a v3.
> Okay!
>
>
> Thanks & regards
> Sumitra

I think one thing worth mentioning in the context of this patch is that 
IIRC kmap_local_page() will block offlining of the mapping CPU until 
kunmap_local(), so while I haven't seen any guidelines around the usage 
of this api for long-held mappings, I figure it's wise to keep the 
mapping duration short, or at least avoid sleeping with a 
kmap_local_page() map active.

I figured, while page compression is probably to be considered "slow" 
it's probably not slow enough to motivate kmap() instead of 
kmap_local_page(), but if anyone feels differently, perhaps it should be 
considered.

With that said, my Reviewed-by: still stands.

/Thomas

> > Thanks!
> > Ira
> >
> > [*] https://lore.kernel.org/all/bcb0a1d2-cd4d-a56f-1ee6-7ccfdd2f7b38@xxxxxxxxxxxxxxx/
> >
> > <quote>
> > Thanks all! I'll just re-send the patch for our CI, since it didn't get
> > picked up automatically (stuck in moderation perhaps), with all r-b tags
> > added and extra line space removed and merge it if results will be green.
> >
> > Regards,
> >
> > Tvrtko
> > </quote>
> >
> >
> > > Thanks & regards
> > > Sumitra
> > >
> > > PS: I am new to the open source vocabulary terms.
> > >
> > > > > 	- Change commit subject and message.
> > > > >
> > > > >   drivers/gpu/drm/i915/i915_gpu_error.c | 4 ++--
> > > > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
> > > > > index f020c0086fbc..bc41500eedf5 100644
> > > > > --- a/drivers/gpu/drm/i915/i915_gpu_error.c
> > > > > +++ b/drivers/gpu/drm/i915/i915_gpu_error.c
> > > > > @@ -1164,9 +1164,9 @@ i915_vma_coredump_create(const struct intel_gt *gt,
> > > > >   
> > > > >   			drm_clflush_pages(&page, 1);
> > > > >   
> > > > > -			s = kmap(page);
> > > > > +			s = kmap_local_page(page);
> > > > >   			ret = compress_page(compress, s, dst, false);
> > > > > -			kunmap(page);
> > > > > +			kunmap_local(s);
> > > > >   
> > > > >   			drm_clflush_pages(&page, 1);
> > > > >   
> > > > > --
> > > > > 2.25.1