Re: [PATCH 2/9] drm/i915: Use kmap_local_page() in gem/i915_gem_pyhs.c

[Zhao Liu <zhao1.liu@linux.intel.com>]

On Sat, Oct 29, 2022 at 03:32:08PM +0200, Fabio M. De Francesco wrote:
> Date: Sat, 29 Oct 2022 15:32:08 +0200
> From: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
> Subject: Re: [PATCH 2/9] drm/i915: Use kmap_local_page() in
>  gem/i915_gem_pyhs.c
> 
> On luned? 17 ottobre 2022 11:37:18 CEST Zhao Liu wrote:
> > From: Zhao Liu <zhao1.liu@intel.com>
> > 
> > The use of kmap_atomic() is being deprecated in favor of
> > kmap_local_page()[1].
> > 
> > The main difference between atomic and local mappings is that local
> > mappings doesn't disable page faults or preemption.
> > 
> > In drm/i915/gem/i915_gem_phys.c, the functions
> > i915_gem_object_get_pages_phys() and i915_gem_object_put_pages_phys()
> > don't need to disable pagefaults and preemption for mapping because of
> > these 2 reasons:
> > 
> > 1. The flush operation is safe for CPU hotplug when preemption is not
> > disabled. In drm/i915/gem/i915_gem_object.c, the functions
> > i915_gem_object_get_pages_phys() and i915_gem_object_put_pages_phys()
> > calls drm_clflush_virt_range() to use CLFLUSHOPT or WBINVD to flush.
> > Since CLFLUSHOPT is global on x86 and WBINVD is called on each cpu in
> > drm_clflush_virt_range(), the flush operation is global and any issue
> > with cpu's being added or removed can be handled safely.
> > 
> > 2. Any context switch caused by preemption or sleep (pagefault may
> > cause sleep) doesn't affect the validity of local mapping.
> > 
> > Therefore, i915_gem_object_get_pages_phys() and
> > i915_gem_object_put_pages_phys() are two functions where the use of
> > kmap_local_page() in place of kmap_atomic() is correctly suited.
> > 
> > Convert the calls of kmap_atomic() / kunmap_atomic() to
> > kmap_local_page() / kunmap_local().
> > 
> 
> I have here the same questions as in 1/9.
> 
> > [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com
> > 
> > Suggested-by: Dave Hansen <dave.hansen@intel.com>
> > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > ---
> > Suggested by credits:
> >   Dave: Referred to his explanation about cache flush.
> >   Ira: Referred to his task document, review comments and explanation about
> >        cache flush.
> >   Fabio: Referred to his boiler plate commit message.
> > ---
> >  drivers/gpu/drm/i915/gem/i915_gem_phys.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_phys.c
> > b/drivers/gpu/drm/i915/gem/i915_gem_phys.c index 0d0e46dae559..d602ba19ecb2 
> 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_phys.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_phys.c
> > @@ -66,10 +66,10 @@ static int i915_gem_object_get_pages_phys(struct 
> drm_i915_gem_object
> > *obj) if (IS_ERR(page))
> >  			goto err_st;
> > 
> > -		src = kmap_atomic(page);
> > +		src = kmap_local_page(page);
> >  		memcpy(dst, src, PAGE_SIZE);
> >  		drm_clflush_virt_range(dst, PAGE_SIZE);
> > -		kunmap_atomic(src);
> > +		kunmap_local(src);
> 
> Please use memcpy_from_page() instead of open coding mapping + memcpy() + 
> unmapping.

Ok.

> 
> > 
> >  		put_page(page);
> >  		dst += PAGE_SIZE;
> > @@ -114,10 +114,10 @@ i915_gem_object_put_pages_phys(struct 
> drm_i915_gem_object *obj,
> >  			if (IS_ERR(page))
> >  				continue;
> > 
> > -			dst = kmap_atomic(page);
> > +			dst = kmap_local_page(page);
> >  			drm_clflush_virt_range(src, PAGE_SIZE);
> >  			memcpy(dst, src, PAGE_SIZE);
> > -			kunmap_atomic(dst);
> > +			kunmap_local(dst);
> 
> For the same reasons said above, memcpy_to_page() should be used here and 
> avoid open coding of three functions.
> 
> Using those helpers forces you to move drm_clflush_virt_range() out of the 
> mapping / un-mapping region. I may be wrong, however I'm pretty sure that the 
> relative positions of each of those call sites is something that cannot be 
> randomly chosen.

I agree. Will use memcpy_to_page().

Thanks,
Zhao

> 
> Thanks,
> 
> Fabio
> 
> > 
> >  			set_page_dirty(page);
> >  			if (obj->mm.madv == I915_MADV_WILLNEED)
> 
> 
>