Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

Daniel Vetter
2020-11-04T16:27:15Z

On Wed, Nov 4, 2020 at 5:21 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Wed, Nov 04, 2020 at 04:54:19PM +0100, Daniel Vetter wrote:
> > I don't really have a box here, but dma_mmap_attrs() and friends to
> > mmap dma_alloc_coherent memory is set up as VM_IO | VM_PFNMAP (it's
> > actually enforced since underneath it uses remap_pfn_range), and
> > usually (except if it's pre-cma carveout) that's just normal struct
> > page backed memory. Sometimes from a cma region (so will be caught by
> > the cma page check), but if you have an iommu to make it
> > device-contiguous, that's not needed.
>
> dma_mmap_* memory may or may not be page backed, but it absolutely
> must not be resolved by get_user_pages and friends as it is special.
> So yes, not being able to get a struct page back from such an mmap is
> a feature.

Yes, that's clear.

What we're discussing is whether gup_fast and pup_fast also obey this,
or fall over and can give you the struct page that's backing the
dma_mmap_* memory. Since the _fast variant doesn't check for
vma->vm_flags, and afaict that's the only thing which closes this gap.
And like you restate, that would be a bit a problem. So where's that
check which Jason&me aren't spotting?
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation