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Date:   Mon, 24 Dec 2018 15:20:59 +0000
From:   Russell King - ARM Linux <linux@armlinux.org.uk>
To:     Souptick Joarder <jrdr.linux@gmail.com>
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Subject: Re: [PATCH v5 0/9] Use vm_insert_range
Message-ID: <20181224152059.GA26090@n2100.armlinux.org.uk>
References: <20181224131841.GA22017@jordon-HP-15-Notebook-PC>
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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/
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