memcgroup: revert swap_state mods

If we're charging rss and we're charging cache, it seems obvious that we
should be charging swapcache - as has been done.  But in practice that
doesn't work out so well: both swapin readahead and swapoff leave the
majority of pages charged to the wrong cgroup (the cgroup that happened to
read them in, rather than the cgroup to which they belong).

(Which is why unuse_pte's GFP_KERNEL while holding pte lock never showed up
as a problem: no allocation was ever done there, every page read being
already charged to the cgroup which initiated the swapoff.)

It all works rather better if we leave the charging to do_swap_page and
unuse_pte, and do nothing for swapcache itself: revert mm/swap_state.c to
what it was before the memory-controller patches.  This also speeds up
significantly a contained process working at its limit: because it no
longer needs to keep waiting for swap writeback to complete.

Is it unfair that swap pages become uncharged once they're unmapped, even
though they're still clearly private to particular cgroups?  For a short
while, yes; but PageReclaim arranges for those pages to go to the end of
the inactive list and be reclaimed soon if necessary.

shmem/tmpfs pages are a distinct case: their charging also benefits from
this change, but their second life on the lists as swapcache pages may
prove more unfair - that I need to check next.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Pavel Emelianov <xemul@openvz.org>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Kirill Korotaev <dev@sw.ru>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: David Rientjes <rientjes@google.com>
Cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>