Swapoff can make good use of a page's anon_vma and index, while it's still
left in swapcache, or once it's brought back in and the first pte mapped back:
unuse_vma go directly to just one page of only those vmas with the same
anon_vma.  And unuse_process can skip any vmas without an anon_vma (extending
the hugetlb check: hugetlb vmas have no anon_vma).

This just hacks in on top of the existing procedure, still going through all
the vmas of all the mms in mmlist.  A more elegant procedure might replace
mmlist by a list of anon_vmas: but that would be more work to implement, with
apparently more overhead in the common paths.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>