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

[Laurent Pinchart]

On Thu, Nov 24, 2022 at 06:24:52PM +0100, 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?

I've taken a trip back in time to v3.14, when b65e64f7ccd4 was merged,
and checked the x86, arm and arm64 dma mapping implementations. They all
call remap_pfn_range() except for arm_iommu_mmap_attrs(), which uses
vm_insert_page(). It was implemented as

int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
			struct page *page)
{
	if (addr < vma->vm_start || addr >= vma->vm_end)
		return -EFAULT;
	if (!page_count(page))
		return -EINVAL;
	if (!(vma->vm_flags & VM_MIXEDMAP)) {
		BUG_ON(down_read_trylock(&vma->vm_mm->mmap_sem));
		BUG_ON(vma->vm_flags & VM_PFNMAP);
		vma->vm_flags |= VM_MIXEDMAP;
	}
	return insert_page(vma, addr, page, vma->vm_page_prot);
}

I don't recall if I just hit that BUG_ON() back then, or if I actually
understood what I was doing :-)

Today in mainline arm_iommu_mmap_attrs() uses vm_map_pages(), which
calls vm_insert_page(), and the BUG_ON() is still there.

> > > 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 König" <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);

-- 
Regards,

Laurent Pinchart