Re: [BK PATCH 2.5] Introduce 64-bit versions of PAGE_{CACHE_,}{MASK,ALIGN}

[Andrew Morton <akpm@zip.com.au>]

Linus Torvalds wrote:
> 
> ...
> Dream on. It's good, and it's not getting removed. The "struct page" is
> size-critical, and also correctness-critical (see above on gcc issues).
> 

Plan B is to remove page->index.

- Replace ->mapping with a pointer to the page's radix tree
  slot.   Use address masking to go from page.radix_tree_slot
  to the radix tree node.

- Store the base index in the radix tree node, use math to
  derive page->index.  Gives 64-bit index without increasing
  the size of struct page. 4 bytes saved.  

- Implement radix_tree_gang_lookup() as previously described.  Use
  this in truncate_inode_pages, invalidate_inode_pages[2], readahead
  and writeback.

- The only thing we now need page.list for is tracking dirty pages.
  Implement a 64-bit dirtiness bitmap in radix_tree_node, propagate
  that up the radix tree so we can efficiently traverse dirty pages
  in a mapping.  This also allows writeback to always write in ascending
  index order.  Remove page->list.  8 bytes saved.

- Few pages use ->private for much.  Hash for it.  4(ish) bytes
  saved.

- Remove ->virtual, do page_address() via a hash.  4(ish) bytes saved.

- Remove the rmap chain (I just broke ptep_to_address() anyway).  4 bytes
  saved.  struct page is now 20 bytes.

There look.  In five minutes I shrunk 24 bytes from the page
structure.  Who said programming was hard?

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