[commit review] vm: add vm_iomap_memory() helper function

[Mathieu Desnoyers]

Hi Linus,

Poking my nose into this new helper, 

> commit b4cbb197c7e7a68dbad0d491242e3ca67420c13e
> Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Date:   Tue Apr 16 13:45:37 2013 -0700
> 
>     vm: add vm_iomap_memory() helper function
[...]     
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2393,6 +2393,53 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
>  }
>  EXPORT_SYMBOL(remap_pfn_range);
>  
> +/**
> + * vm_iomap_memory - remap memory to userspace
> + * @vma: user vma to map to
> + * @start: start of area
> + * @len: size of area
> + *
> + * This is a simplified io_remap_pfn_range() for common driver use. The
> + * driver just needs to give us the physical memory range to be mapped,
> + * we'll figure out the rest from the vma information.
> + *
> + * NOTE! Some drivers might want to tweak vma->vm_page_prot first to get
> + * whatever write-combining details or similar.
> + */
> +int vm_iomap_memory(struct vm_area_struct *vma, phys_addr_t start, unsigned long len)
> +{
> +	unsigned long vm_len, pfn, pages;
> +
> +	/* Check that the physical memory area passed in looks valid */
> +	if (start + len < start)

is len == 0 accepted ?

> +		return -EINVAL;
> +	/*
> +	 * You *really* shouldn't map things that aren't page-aligned,
> +	 * but we've historically allowed it because IO memory might
> +	 * just have smaller alignment.
> +	 */
> +	len += start & ~PAGE_MASK;
> +	pfn = start >> PAGE_SHIFT;
> +	pages = (len + ~PAGE_MASK) >> PAGE_SHIFT;
> +	if (pfn + pages < pfn)
> +		return -EINVAL;
> +
> +	/* We start the mapping 'vm_pgoff' pages into the area */
> +	if (vma->vm_pgoff > pages)

This seems to accept vma->vm_pgoff == pages, which would leave exactly 0
backing pages. Is this expected ?

> +		return -EINVAL;
> +	pfn += vma->vm_pgoff;
> +	pages -= vma->vm_pgoff;
> +
> +	/* Can we fit all of the mapping? */
> +	vm_len = vma->vm_end - vma->vm_start;
> +	if (vm_len >> PAGE_SHIFT > pages)

Is it possible that we get a situation where vm_end and vm_start are not
aligned on page addresses ? If it's allowed, then this test may succeed
even though the range requires more than "pages" backing pages. This
might be an issue since the first thing remap_pfn_range() does with its
"size" argument is to PAGE_ALIGN() it.

Thoughts ?

Thanks,

Mathieu

> +		return -EINVAL;
> +
> +	/* Ok, let it rip */
> +	return io_remap_pfn_range(vma, vma->vm_start, pfn, vm_len, vma->vm_page_prot);
> +}
> +EXPORT_SYMBOL(vm_iomap_memory);
> +
>  static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
>  				     unsigned long addr, unsigned long end,
>  				     pte_fn_t fn, void *data)


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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