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SubjectRe: [PATCH v4 05/17] drm: add mmap() to drm_gem_object_funcs
  Hi,

> > + vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
> > + vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
> > + vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
> > + }
>
> Totally unrelated discussion around HMM lead me to a bit a chase, and
> realizing that we probably want a
>
> WARN_ON(!(vma->vm_flags & VM_SPECIAL));
>
> here, to make sure drivers set at least one of the "this is a special
> vma, don't try to treat it like pagecache/anon memory". Just to be on
> the safe side. Maybe we also want to keep the entire vma->vm_flags
> assignment in the common code, but at least the WARN_ON would be a
> good check I think. Maybe also check for VM_DONTEXPAND while at it

Hmm. VM_SPECIAL is this:

#define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP)

Requiring VM_DONTEXPAND makes sense for sure. Dunno about the other
ones. For drm_gem_vram_helper VM_IO + VM_PFNMAP are needed.

But we also have drm_gem_shmem_helper which backs objects with normal
pages. In fact drm_gem_shmem_mmap does this:

/* VM_PFNMAP was set by drm_gem_mmap() */
vma->vm_flags &= ~VM_PFNMAP;
vma->vm_flags |= VM_MIXEDMAP;

include/linux/mm.h isn't very helpful in explaining how VM_MIXEDMAP
should be used, only saying can be both "struct page" and pfnmap, so I'm
not sure why VM_MIXEDMAP is set here, it should always be "struct page"
for shmem, no?

cheers,
Gerd

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