Re: [PATCH v3 3/5] drm/i915/gem: Pull out size validation into a separate function

Hi Janusz,

On 2026-07-15 at 17:21:39 +0200, Janusz Krzysztofik wrote:
> Hi Krzysztof,
> 
> On Mon, 2026-07-13 at 09:58 +0000, Krzysztof Karas wrote:
> > shmem_sg_alloc_table is a very large and hard to read function,
> > so reduce the number of operations it is responsible for by
> > placing "size" validation in a new helper.
> > 
> > Signed-off-by: Krzysztof Karas <[email protected]>
> > ---
> > v3:
> >  * Split refactoring and put it after the fix in shmem folio
> >   counting suggested by Andi.
> > 
> >  drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 29 ++++++++++++++++-------
> >  1 file changed, 20 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > index 0011d76f5b8c..4a61b012fb6f 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > @@ -62,6 +62,22 @@ void shmem_sg_free_table(struct sg_table *st, struct address_space *mapping,
> >  	sg_free_table(st);
> >  }
> >  
> > +static int validate_size(size_t size, unsigned int page_count,
> > +			 struct intel_memory_region *mr)
> > +{
> > +	if (overflows_type(size / PAGE_SIZE, page_count))
> > +		return -E2BIG;
> > +
> > +	/*
> > +	 * If there's no chance of allocating enough pages for the whole
> > +	 * object, bail early.
> > +	 */
> > +	if (size > resource_size(&mr->region))
> > +		return -ENOMEM;
> > +
> > +	return 0;
> > +}
> > +
> >  int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
> >  			 size_t size, struct intel_memory_region *mr,
> >  			 struct address_space *mapping,
> > @@ -77,16 +93,11 @@ int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
> >  	unsigned long i;
> >  	int ret;
> >  
> > -	if (overflows_type(size / PAGE_SIZE, page_count))
> > -		return -E2BIG;
> > -
> >  	page_count = size / PAGE_SIZE;
> 
> Verifying if page_count can accommodate a result before assigning that
> result to it looked more correctly to me.  Since overflows_type() doesn't
> look at the variable's value, only its type, I think you could postpone 
> page_count initialization and pass its pointer to your helper to preserve 
> that more reasonable order of operations.
Yeah, you are right, it would make more sense to validate it
before assignment. I'll do that in the next version.

> 
> Thanks,
> Janusz
> 
> > -	/*
> > -	 * If there's no chance of allocating enough pages for the whole
> > -	 * object, bail early.
> > -	 */
> > -	if (size > resource_size(&mr->region))
> > -		return -ENOMEM;
> > +
> > +	ret = validate_size(size, page_count, mr);
> > +	if (ret < 0)
> > +		return ret;
> >  
> >  	if (sg_alloc_table(st, page_count, GFP_KERNEL | __GFP_NOWARN))
> >  		return -ENOMEM;

-- 
Best Regards,
Krzysztof