If we're thinking about shmem scalability...  isn't it silly that each shmem
object is added to the shmem_inodes list on creation, and removed on deletion,
yet the only use for that list is in swapoff (shmem_unuse)?

Call it shmem_swaplist; shmem_writepage add inode to swaplist when first swap
allocated (usually never); shmem_delete_inode remove inode from the list after
truncating (if called before, inode could be re-added to it).

Inode can remain on the swaplist after all its pages are swapped back in, just
be lazy about it; but if shmem_unuse finds swapped count now 0, save itself
time by then removing that inode from the swaplist.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>