linux-kernel - Re: [PATCH 2/8] drm/gem: Introduce drm_gem_get_unmapped

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Message-ID: <22680961-1a51-469a-93df-ee9a63b3fd66@collabora.com>
Date: Tue, 30 Sep 2025 18:42:07 +0200
From: Loïc Molinari <loic.molinari@...labora.com>
To: Boris Brezillon <boris.brezillon@...labora.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Jani Nikula <jani.nikula@...ux.intel.com>,
 Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
 Rodrigo Vivi <rodrigo.vivi@...el.com>, Tvrtko Ursulin
 <tursulin@...ulin.net>, Rob Herring <robh@...nel.org>,
 Steven Price <steven.price@....com>, Liviu Dudau <liviu.dudau@....com>,
 Melissa Wen <mwen@...lia.com>, Maíra Canal
 <mcanal@...lia.com>, Hugh Dickins <hughd@...gle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Al Viro
 <viro@...iv.linux.org.uk>, Mikołaj Wasiak
 <mikolaj.wasiak@...el.com>, Christian Brauner <brauner@...nel.org>,
 Nitin Gote <nitin.r.gote@...el.com>, Andi Shyti
 <andi.shyti@...ux.intel.com>, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
 linux-mm@...ck.org, kernel@...labora.com
Subject: Re: [PATCH 2/8] drm/gem: Introduce drm_gem_get_unmapped_area() fop

On 30/09/2025 18:29, Boris Brezillon wrote:
> On Tue, 30 Sep 2025 18:09:37 +0200
> Loïc Molinari <loic.molinari@...labora.com> wrote:
> 
>> On 30/09/2025 12:30, Boris Brezillon wrote:
>>> On Mon, 29 Sep 2025 22:03:10 +0200
>>>
>>> Loïc Molinari <loic.molinari@...labora.com> 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.

Yes, sounds like a very good point. I'll remove the ifdef and forward to 
the shmem layer unconditionally.

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