Re: [Intel-gfx] [PATCH] drm/i915: Call page_address() on page acquired with GFP_KERNEL flag

[Sumitra Sharma <sumitraartsy@gmail.com>]

On Wed, Jun 14, 2023 at 05:30:25PM +0200, Thomas Hellström (Intel) wrote:
> 
> On 6/14/23 15:22, Tvrtko Ursulin wrote:
> > 
> > On 14/06/2023 13:35, Sumitra Sharma wrote:
> > > Pages allocated with GFP_KERNEL cannot come from Highmem.
> > > That is why there is no need to call kmap() on them.
> > 
> > Are you sure it is GFP_KERNEL backed and not tmpfs? I am not sure myself
> > so let me copy Matt and Thomas if they happen to know off hand.
>

Hello,

Yes it is true that the pages have not been acquired using the GFP_KERNEL.

I confused the allocation of the struct 'i915_vma_resource' tracking the 
pages with the allocation of the pages themselves.

This was noted by my mentor Ira Weiny <ira.weiny@intel.com>.

> It looks to me these are shmem pages or TTM pages. Both could be highmem. So
> I think kmap is the correct choice here.
> 

However, the kmap() will not be the correct choice here and kmap_local_page()
must be used instead. I have created a v2 patch for the same
https://lore.kernel.org/lkml/20230617180420.GA410966@sumitra.com/

Thank you for helping me.

Regards
Sumitra

> /Thomas
>
> 
> 
> 
> > 
> > Regards,
> > 
> > Tvrtko
> > 
> > > Therefore, don't call kmap() on the page coming from
> > > vma_res->bi.pages using for_each_sgt_page() in
> > > i915_vma_coredump_create().
> > > 
> > > Use a plain page_address() to get the kernel address instead.
> > > 
> > > Signed-off-by: Sumitra Sharma <sumitraartsy@gmail.com>
> > > ---
> > >   drivers/gpu/drm/i915/i915_gpu_error.c | 3 +--
> > >   1 file changed, 1 insertion(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c
> > > b/drivers/gpu/drm/i915/i915_gpu_error.c
> > > index f020c0086fbc..6f51cb4fc55c 100644
> > > --- a/drivers/gpu/drm/i915/i915_gpu_error.c
> > > +++ b/drivers/gpu/drm/i915/i915_gpu_error.c
> > > @@ -1164,9 +1164,8 @@ i915_vma_coredump_create(const struct intel_gt
> > > *gt,
> > >                 drm_clflush_pages(&page, 1);
> > >   -            s = kmap(page);
> > > +            s = page_address(page);
> > >               ret = compress_page(compress, s, dst, false);
> > > -            kunmap(page);
> > >                 drm_clflush_pages(&page, 1);