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Mon, 10 Nov 2025 15:32:01 +0100 (CET) Message-ID: <313f7e46-e1ce-46de-ab81-d842c07485df@collabora.com> Date: Mon, 10 Nov 2025 15:32:01 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 04/12] drm/gem: Introduce drm_gem_get_unmapped_area() fop To: Hugh Dickins Cc: Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Tvrtko Ursulin , Boris Brezillon , Rob Herring , Steven Price , Liviu Dudau , Melissa Wen , =?UTF-8?Q?Ma=C3=ADra_Canal?= , Baolin Wang , Andrew Morton , Al Viro , =?UTF-8?Q?Miko=C5=82aj_Wasiak?= , Christian Brauner , Nitin Gote , Andi Shyti , Jonathan Corbet , Christopher Healy , Matthew Wilcox , Bagas Sanjaya , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, kernel@collabora.com References: <20251021113049.17242-1-loic.molinari@collabora.com> <20251021113049.17242-5-loic.molinari@collabora.com> Content-Language: fr From: =?UTF-8?Q?Lo=C3=AFc_Molinari?= Organization: Collabora Ltd In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: r6sqhraroex4ecu8d4wotssqe7tqz49d X-Rspam-User: X-Rspamd-Queue-Id: 4E8B1140010 X-Rspamd-Server: rspam10 X-HE-Tag: 1762785124-623223 X-HE-Meta: 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 ZMd6btjt 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Hugh, On 27/10/2025 12:38, Hugh Dickins wrote: > On Tue, 21 Oct 2025, Loïc Molinari wrote: > >> mmap() calls on the DRM file pointer currently always end up using >> mm_get_unmapped_area() to get a free mapping region. On builds with >> CONFIG_TRANSPARENT_HUGEPAGE enabled, this isn't ideal for GEM objects >> backed by shmem buffers on mountpoints setting the 'huge=' option >> because it can't correctly figure out the potentially huge address >> alignment required. >> >> This commit introduces the drm_gem_get_unmapped_area() function which >> is meant to be used as a get_unmapped_area file operation on the DRM >> file pointer to lookup GEM objects based on their fake offsets and get >> a properly aligned region by calling shmem_get_unmapped_area() with >> the right file pointer. If a GEM object isn't available at the given >> offset or if the caller isn't granted access to it, the function falls >> back to mm_get_unmapped_area(). >> >> This also makes drm_gem_get_unmapped_area() part of the default GEM >> file operations so that all the DRM drivers can benefit from more >> efficient mappings thanks to the huge page fault handler introduced in >> previous commit 'drm/shmem-helper: Add huge page fault handler'. >> >> The shmem_get_unmapped_area() function needs to be exported so that >> it can be used from the DRM subsystem. >> >> v3: >> - add missing include: 'linux/sched/mm.h' >> - forward to shmem layer in builds with CONFIG_TRANSPARENT_HUGEPAGE=n >> >> Signed-off-by: Loïc Molinari > > Seems reasonable, but a couple of minor remarks below. > >> --- >> drivers/gpu/drm/drm_gem.c | 107 ++++++++++++++++++++++++++++++-------- >> include/drm/drm_gem.h | 4 ++ >> mm/shmem.c | 1 + >> 3 files changed, 90 insertions(+), 22 deletions(-) >> >> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c >> index a1a9c828938b..a98d5744cc6c 100644 >> --- a/drivers/gpu/drm/drm_gem.c >> +++ b/drivers/gpu/drm/drm_gem.c >> @@ -36,6 +36,7 @@ >> #include >> #include >> #include >> +#include >> #include >> #include >> #include >> @@ -1187,36 +1188,27 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size, >> } >> EXPORT_SYMBOL(drm_gem_mmap_obj); >> >> -/** >> - * drm_gem_mmap - memory map routine for GEM objects >> - * @filp: DRM file pointer >> - * @vma: VMA for the area to be mapped >> - * >> - * If a driver supports GEM object mapping, mmap calls on the DRM file >> - * descriptor will end up here. >> - * >> - * Look up the GEM object based on the offset passed in (vma->vm_pgoff will >> - * contain the fake offset we created when the GTT map ioctl was called on >> - * the object) and map it with a call to drm_gem_mmap_obj(). >> - * >> - * If the caller is not granted access to the buffer object, the mmap will fail >> - * with EACCES. Please see the vma manager for more information. >> +/* >> + * Look up a GEM object in offset space based on the exact start address. The >> + * caller must be granted access to the object. Returns a GEM object on success >> + * or a negative error code on failure. The returned GEM object needs to be >> + * released with drm_gem_object_put(). >> */ >> -int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma) >> +static struct drm_gem_object * >> +drm_gem_object_lookup_from_offset(struct file *filp, unsigned long start, >> + unsigned long pages) >> { >> struct drm_file *priv = filp->private_data; >> struct drm_device *dev = priv->minor->dev; >> struct drm_gem_object *obj = NULL; >> struct drm_vma_offset_node *node; >> - int ret; >> >> if (drm_dev_is_unplugged(dev)) >> - return -ENODEV; >> + return ERR_PTR(-ENODEV); >> >> drm_vma_offset_lock_lookup(dev->vma_offset_manager); >> node = drm_vma_offset_exact_lookup_locked(dev->vma_offset_manager, >> - vma->vm_pgoff, >> - vma_pages(vma)); >> + start, pages); >> if (likely(node)) { >> obj = container_of(node, struct drm_gem_object, vma_node); >> /* >> @@ -1235,14 +1227,85 @@ int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma) >> drm_vma_offset_unlock_lookup(dev->vma_offset_manager); >> >> if (!obj) >> - return -EINVAL; >> + return ERR_PTR(-EINVAL); >> >> if (!drm_vma_node_is_allowed(node, priv)) { >> drm_gem_object_put(obj); >> - return -EACCES; >> + return ERR_PTR(-EACCES); >> } >> >> - ret = drm_gem_mmap_obj(obj, drm_vma_node_size(node) << PAGE_SHIFT, >> + return obj; >> +} >> + >> +/** >> + * drm_gem_get_unmapped_area - get memory mapping region routine for GEM objects >> + * @filp: DRM file pointer >> + * @uaddr: User address hint >> + * @len: Mapping length >> + * @pgoff: Offset (in pages) >> + * @flags: Mapping flags >> + * >> + * If a driver supports GEM object mapping, before ending up in drm_gem_mmap(), >> + * mmap calls on the DRM file descriptor will first try to find a free linear >> + * address space large enough for a mapping. Since GEM objects are backed by >> + * shmem buffers, this should preferably be handled by the shmem virtual memory >> + * filesystem which can appropriately align addresses to huge page sizes when >> + * needed. >> + * >> + * Look up the GEM object based on the offset passed in (vma->vm_pgoff will >> + * contain the fake offset we created) and call shmem_get_unmapped_area() with >> + * the right file pointer. >> + * >> + * If a GEM object is not available at the given offset or if the caller is not >> + * granted access to it, fall back to mm_get_unmapped_area(). >> + */ >> +unsigned long drm_gem_get_unmapped_area(struct file *filp, unsigned long uaddr, >> + unsigned long len, unsigned long pgoff, >> + unsigned long flags) >> +{ >> + 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; >> +} >> +EXPORT_SYMBOL(drm_gem_get_unmapped_area); > > Not something I'll make an issue of, but this does look rather like > a drm EXPORT_SYMBOL() of a shmem EXPORT_SYMBOL_GPL(). > > Not your intention, I think, and a quick look around suggests some > inconsistency as to whether symbols here are exported _GPL() or not. > > Maybe there's good (historical?) reason for which is which, or > maybe it's something maintainers would like to clean up one day. > > Please make this one EXPORT_SYMBOL_GPL() if you can. > >> + >> +/** >> + * drm_gem_mmap - memory map routine for GEM objects >> + * @filp: DRM file pointer >> + * @vma: VMA for the area to be mapped >> + * >> + * If a driver supports GEM object mapping, mmap calls on the DRM file >> + * descriptor will end up here. >> + * >> + * Look up the GEM object based on the offset passed in (vma->vm_pgoff will >> + * contain the fake offset we created) and map it with a call to >> + * drm_gem_mmap_obj(). >> + * >> + * If the caller is not granted access to the buffer object, the mmap will fail >> + * with EACCES. Please see the vma manager for more information. >> + */ >> +int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma) >> +{ >> + struct drm_gem_object *obj; >> + int ret; >> + >> + obj = drm_gem_object_lookup_from_offset(filp, vma->vm_pgoff, >> + vma_pages(vma)); >> + if (IS_ERR(obj)) >> + return PTR_ERR(obj); >> + >> + ret = drm_gem_mmap_obj(obj, >> + drm_vma_node_size(&obj->vma_node) << PAGE_SHIFT, >> vma); >> >> drm_gem_object_put(obj); >> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h >> index 8d48d2af2649..7c8bd67d087c 100644 >> --- a/include/drm/drm_gem.h >> +++ b/include/drm/drm_gem.h >> @@ -469,6 +469,7 @@ struct drm_gem_object { >> .poll = drm_poll,\ >> .read = drm_read,\ >> .llseek = noop_llseek,\ >> + .get_unmapped_area = drm_gem_get_unmapped_area,\ >> .mmap = drm_gem_mmap, \ >> .fop_flags = FOP_UNSIGNED_OFFSET >> >> @@ -506,6 +507,9 @@ void drm_gem_vm_close(struct vm_area_struct *vma); >> int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size, >> struct vm_area_struct *vma); >> int drm_gem_mmap(struct file *filp, struct vm_area_struct *vma); >> +unsigned long drm_gem_get_unmapped_area(struct file *filp, unsigned long uaddr, >> + unsigned long len, unsigned long pgoff, >> + unsigned long flags); >> >> /** >> * drm_gem_object_get - acquire a GEM buffer object reference >> diff --git a/mm/shmem.c b/mm/shmem.c >> index b9081b817d28..612218fc95cb 100644 >> --- a/mm/shmem.c >> +++ b/mm/shmem.c >> @@ -2851,6 +2851,7 @@ unsigned long shmem_get_unmapped_area(struct file *file, >> return addr; >> return inflated_addr; >> } >> +EXPORT_SYMBOL_GPL(shmem_get_unmapped_area); > > As you have it, that export comes under #ifdef CONFIG_SHMEM. > > I know parts of drm do "select SHMEM" these days, but you might not > be covered by those: maybe you need a "select SHMEM" in the relevant > Kconfig, or maybe you prefer to duplicate that export line after the > later !CONFIG_SHMEM definitiion of shmem_get_unmapped_area(). > > Or better, make no export here at all, but drm_gem_get_unmapped_area() > use obj->filp->f_op->get_unmapped_area()? This is indeed better, thanks. I've just proposed a v6 doing so. >> >> #ifdef CONFIG_NUMA >> static int shmem_set_policy(struct vm_area_struct *vma, struct mempolicy *mpol) >> -- >> 2.47.3 Loïc