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From:   Souptick Joarder <jrdr.linux@gmail.com>
Date:   Wed, 26 Dec 2018 19:11:57 +0530
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Subject: Re: [PATCH v5 0/9] Use vm_insert_range
To:     Russell King - ARM Linux <linux@armlinux.org.uk>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
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On Mon, Dec 24, 2018 at 8:51 PM Russell King - ARM Linux
<linux@armlinux.org.uk> 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.