On Mon, Dec 24, 2018 at 8:51 PM Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxx> wrote:
>
> Having discussed with Matthew offlist, I think we've come to the
> following conclusion - there's a number of drivers that buggily
> ignore vm_pgoff.
>
> So, what I proposed is:
>
> static int __vm_insert_range(struct vm_struct *vma, struct page *pages,
> size_t num, unsigned long offset)
> {
> unsigned long count = vma_pages(vma);
> unsigned long uaddr = vma->vm_start;
> int ret;
>
> /* Fail if the user requested offset is beyond the end of the object */
> if (offset > num)
> return -ENXIO;
>
> /* Fail if the user requested size exceeds available object size */
> if (count > num - offset)
> return -ENXIO;
>
> /* Never exceed the number of pages that the user requested */
> for (i = 0; i < count; i++) {
> ret = vm_insert_page(vma, uaddr, pages[offset + i]);
> if (ret < 0)
> return ret;
> uaddr += PAGE_SIZE;
> }
>
> return 0;
> }
>
> /*
> * Maps an object consisting of `num' `pages', catering for the user's
> * requested vm_pgoff
> */
> int vm_insert_range(struct vm_struct *vma, struct page *pages, size_t num)
> {
> return __vm_insert_range(vma, pages, num, vma->vm_pgoff);
> }
>
> /*
> * Maps a set of pages, always starting at page[0]
> */
> int vm_insert_range_buggy(struct vm_struct *vma, struct page *pages, size_t num)
> {
> return __vm_insert_range(vma, pages, num, 0);
> }
>
> With this, drivers such as iommu/dma-iommu.c can be converted thusly:
>
> int iommu_dma_mmap(struct page **pages, size_t size, struct vm_area_struct *vma+)
> {
> - unsigned long uaddr = vma->vm_start;
> - unsigned int i, count = PAGE_ALIGN(size) >> PAGE_SHIFT;
> - int ret = -ENXIO;
> -
> - for (i = vma->vm_pgoff; i < count && uaddr < vma->vm_end; i++) {
> - ret = vm_insert_page(vma, uaddr, pages[i]);
> - if (ret)
> - break;
> - uaddr += PAGE_SIZE;
> - }
> - return ret;
> + return vm_insert_range(vma, pages, PAGE_ALIGN(size) >> PAGE_SHIFT);
> }
>
> and drivers such as firewire/core-iso.c:
>
> int fw_iso_buffer_map_vma(struct fw_iso_buffer *buffer,
> struct vm_area_struct *vma)
> {
> - unsigned long uaddr;
> - int i, err;
> -
> - uaddr = vma->vm_start;
> - for (i = 0; i < buffer->page_count; i++) {
> - err = vm_insert_page(vma, uaddr, buffer->pages[i]);
> - if (err)
> - return err;
> -
> - uaddr += PAGE_SIZE;
> - }
> -
> - return 0;
> + return vm_insert_range_buggy(vma, buffer->pages, buffer->page_count);
> }
>
> and this gives us something to grep for to find these buggy drivers.
>
> Now, this may not look exactly equivalent, but if you look at
> fw_device_op_mmap(), buffer->page_count is basically vma_pages(vma)
> at this point, which means this should be equivalent.
>
> We _could_ then at a later date "fix" these drivers to behave according
> to the normal vm_pgoff offsetting simply by removing the _buggy suffix
> on the function name... and if that causes regressions, it gives us an
> easy way to revert (as long as vm_insert_range_buggy() remains
> available.)
>
> In the case of firewire/core-iso.c, it currently ignores the mmap offset
> entirely, so making the above suggested change would be tantamount to
> causing it to return -ENXIO for any non-zero mmap offset.
>
> IMHO, this approach is way simpler, and easier to get it correct at
> each call site, rather than the current approach which seems to be
> error-prone.
Thanks Russell.
I will drop this patch series and rework on it as suggested.
