Re: [PATCH 2/8] drm/gem: Introduce drm_gem_get_unmapped_area() fop

[Boris Brezillon]

On Tue, 30 Sep 2025 18:09:37 +0200
Loïc Molinari <loic.molinari@xxxxxxxxxxxxx> wrote:

> On 30/09/2025 12:30, Boris Brezillon wrote:
> > On Mon, 29 Sep 2025 22:03:10 +0200
> >
> > Loïc Molinari <loic.molinari@xxxxxxxxxxxxx> wrote:  
> >> +unsigned long drm_gem_get_unmapped_area(struct file *filp, unsigned long uaddr,
> >> +					unsigned long len, unsigned long pgoff,
> >> +					unsigned long flags)
> >> +{
> >> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >> +	struct drm_gem_object *obj;
> >> +	unsigned long ret;
> >> +
> >> +	obj = drm_gem_object_lookup_from_offset(filp, pgoff, len >> PAGE_SHIFT);
> >> +	if (IS_ERR(obj))
> >> +		return mm_get_unmapped_area(current->mm, filp, uaddr, len, 0,
> >> +					    flags);
> >> +
> >> +	ret = shmem_get_unmapped_area(obj->filp, uaddr, len, 0, flags);
> >> +
> >> +	drm_gem_object_put(obj);
> >> +
> >> +	return ret;
> >> +#else
> >> +	return mm_get_unmapped_area(current->mm, filp, uaddr, len, 0, flags);  
> > 
> > Looks like the above code covers the non-THP case too, do we really need
> > to specialize for !CONFIG_TRANSPARENT_HUGEPAGE here?  
> 
> It does cover the !CONFIG_TRANSPARENT_HUGEPAGE case 
> (shmem_get_unmapped_area() would just call and return the 
> mm_get_unmapped_area() address) but the idea here is to avoid the GEM 
> object lookup cost by calling mm_get_unmapped_area() directly.

I'd expect the extra GEM lookup to be negligible compared to the overall
mmap() operation to be honest, but I guess if we really want to avoid
the overhead, we could still write it without this ifdef.

	if (!IS_ENABLED(TRANSPARENT_HUGEPAGE))
		return mm_get_unmapped_area(current->mm, filp, uaddr,
					    len, 0, flags);

	...

My main concern is that shmem_get_unmapped_area() evolves with more
!TRANSPARENT_HUGEPAGE cases, and by calling mm_get_unmapped_area()
directly, we miss the opportunity to get optimizations for these cases,
just like we missed them by not forwarding the ->get_unmapped_area()
requests to the shmem layer so far.

> 
> >> +#endif
> >> +}
> >> +EXPORT_SYMBOL(drm_gem_get_unmapped_area);  
> 
> Loïc