Re: [Xen-devel] [PATCH v5 0/9] Use vm_insert_range

[Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx>]

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.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
According to speedtest.net: 11.9Mbps down 500kbps up

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