[PROJECT] clean up swapcache use of struct page

[Matthew Wilcox <willy@infradead.org>]

This would be a good project for someone with a little experience and
a lot of attention to detail.

The struct page is probably the most abused data structure in the kernel,
and for good reason.  But some of the abuse is unnecessary ... a mere
historical accident that would be better fixed.

Page cache pages use page->mapping and page->index to indicate which file
the page belongs to and where in that file it is.  page->private may be
used by the filesystem for its own purposes (eg buffer heads).

Anonymous pages use page->mapping to point to the anon VMA they belong
to and page->index to record the offset within the VMA.  Then, if they
are also part of the swap cache, they use page->private to record both
the offset within the swap device and the index of the page within the
swap device.

Then we get abominations like:

static inline pgoff_t page_index(struct page *page)
{
        if (unlikely(PageSwapCache(page)))
                return __page_file_index(page);
        return page->index;
}

My modest proposal for deleting the first two lines of that function is
to first switch the uses of page->private and page->index for anonymous
pages.  Then move the swp_type() back from page->index to page->private
again [1].

I am willing to review patches and provide feedback.  I can go into more
detail about how I think this should be tackled if there's interest.
Also, if you know more than I do about the MM and think this is a bad
idea, please do say ;-)

This is going to be a tough project because there are a lot of
rarely-tested paths which directly reference (eg) page->index, and they
might be talking about a page cache page or a swap page.  This is not
a simple Coccinelle script.

[1] We have enough bits to do this; on a 32-bit machine, we can at most
have a VMA which covers 4GB memory and with a 4kB page size, that's only
20 bits needed to encode all possible offsets within a VMA).