[PATCH] drm/dma-helpers: Don't change vma flags

[Robin Murphy]

On 2022-11-24 17:24, Daniel Vetter wrote:
> On Thu, Nov 24, 2022 at 11:11:21AM +0000, Robin Murphy wrote:
>> On 2022-11-23 17:28, Daniel Vetter wrote:
>>> This code was added in b65e64f7ccd4 ("drm/cma: Use
>>> dma_mmap_writecombine() to mmap buffer"), but does not explain why
>>> it's needed.
>>>
>>> It should be entirely unnecessary, because remap_pfn_range(), which is
>>> what the various dma_mmap functiosn are built on top of, does already
>>> unconditionally adjust the vma flags:
>>
>> Not all dma_mmap_*() implementations use remap_pfn_range() though. For
>> instance on ARM where one set of DMA ops uses vm_map_pages(), but AFAICS not
>> all the relevant drivers would set VM_MIXEDMAP to prevent reaching the
>> BUG_ON(vma->vm_flags & VM_PFNMAP) in there.
> 
> Uh a dma_mmap which does not use VM_PFNMAP has pretty good chances of
> being busted, since that allows get_user_pages on these memory
> allocations. And I'm really not sure that's a bright idea ...
> 
> Can you please point me at these dma-ops so that I can try and understand
> what they're trying to do?

See arm_iommu_mmap_attrs(), but also one of the paths in 
iommu_dma_mmap() for both arm64 and x86. These aren't using 
remap_pfn_range() because they're mapping a potentially-disjoint set of 
arbitrary pages, not a single physically-contiguous range. And for the 
avoidance of doubt, yes, in those cases they will always be real kernel 
pages. dma_mmap_attrs() can be relied upon to do the right thing for 
whatever dma_alloc_attrs() did; what isn't reliable is trying to 
second-guess from outside exactly what that might be.

I forgot to mention also that removing the VM_DONTEXPAND line will 
seemingly just reintroduce the annoying warning spam for which we added 
it in the first place (and 59f39bfa6553 does document this same reasoning).

Thanks,
Robin.

> -Daniel
> 
>>
>> Robin.
>>
>>> https://elixir.bootlin.com/linux/v6.1-rc6/source/mm/memory.c#L2518
>>>
>>> More importantly, it does uncondtionally set VM_PFNMAP, so clearing
>>> that does not make much sense.
>>>
>>> Patch motived by discussions around enforcing VM_PFNMAP semantics for
>>> all dma-buf users, where Thomas asked why dma helpers will work with
>>> that dma_buf_mmap() contract.
>>>
>>> References: https://lore.kernel.org/dri-devel/5c3c8d4f-2c06-9210-b00a-4d0ff6f6fbb7@suse.de/
>>> Cc: Laurent Pinchart <laurent.pinchart+renesas at ideasonboard.com>
>>> Cc: Dave Airlie <airlied at redhat.com>
>>> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
>>> Cc: Maxime Ripard <mripard at kernel.org>
>>> Cc: Thomas Zimmermann <tzimmermann at suse.de>
>>> Cc: Sumit Semwal <sumit.semwal at linaro.org>
>>> Cc: "Christian K�nig" <christian.koenig at amd.com>
>>> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
>>> ---
>>>    drivers/gpu/drm/drm_gem_dma_helper.c | 7 ++-----
>>>    1 file changed, 2 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_gem_dma_helper.c b/drivers/gpu/drm/drm_gem_dma_helper.c
>>> index 1e658c448366..637a5cc62457 100644
>>> --- a/drivers/gpu/drm/drm_gem_dma_helper.c
>>> +++ b/drivers/gpu/drm/drm_gem_dma_helper.c
>>> @@ -525,13 +525,10 @@ int drm_gem_dma_mmap(struct drm_gem_dma_object *dma_obj, struct vm_area_struct *
>>>    	int ret;
>>>    	/*
>>> -	 * Clear the VM_PFNMAP flag that was set by drm_gem_mmap(), and set the
>>> -	 * vm_pgoff (used as a fake buffer offset by DRM) to 0 as we want to map
>>> -	 * the whole buffer.
>>> +	 * Set the vm_pgoff (used as a fake buffer offset by DRM) to 0 as we
>>> +	 * want to map the whole buffer.
>>>    	 */
>>>    	vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
>>> -	vma->vm_flags &= ~VM_PFNMAP;
>>> -	vma->vm_flags |= VM_DONTEXPAND;
>>>    	if (dma_obj->map_noncoherent) {
>>>    		vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
>