[PATCH v3] Make frontswap+cleancache and its friend be modularized.

[Konrad Rzeszutek Wilk]

Greg, I am CC-ing you here since there is one patch that touches
the staging tree:
 [PATCH 02/11] frontswap: Make frontswap_init use a pointer for the

I am hoping that for #2 you are OK ACK-ing - it is a simple fix that ought
to have been a long time ago.

I am hoping that these patches are ready for the v3.9 merge window and
would very much appreciate feedback. If I had missed somebodies comments
- please remind me as these last two weeks have been quite busy for me.

Back to the description:

Parts of this patch have been posted in the post (way back in November), but
this patchset expanded it a bit. The goal of the patches is to make the
different frontswap/cleancache API backends be modules - and load way way after
the swap system (or filesystem) has been initialized. Naturally one can still
build the frontswap+cleancache backend devices in the kernel. The next goal
(after these patches) is to also be able to unload the backend drivers - but
that places some interesting requirements to "reload" the swap device with
swap pages (don't need to worry that much about cleancache as it is a "secondary"
cache and can be dumped). Seth had posted some patches for that in the zswap
backend - and they could be more generally repurporsed.

Anyhow, I did not want to lose the authorship of some of the patches so I
didn't squash the ones that were made by Dan and mine. I can do it for review
if it would make it easier, but from my recollection on how Linus likes things
run he would prefer to keep the history (even the kludge parts).

The general flow prior to these patches was [I am concentrating on the
frontswap here, but the cleancache is similar, just s/swapon/mount/]:

 1) kernel inits frontswap_init
 2) kernel inits zcache (or some other backend)
 3) user does swapon /dev/XX and the writes to the swap disk end up in
    frontswap and then in the backend.

With the module loading, the 1) is still part of the bootup, but the
2) or 3) can be run at anytime. This means one could load the backend
_after_ the swap disk has been initialized and running along. Or
_before_ the swap disk has been setup - but that is similar to the
existing case so not that exciting.

To deal with that scenario the frontswap keeps an queue (actually an atomic
bitmap of the swap disks that have been init) and when the backend registers -
frontswap runs the backend init on the queued up swap disks.

The interesting thing is that we can be to certain degree racy when the
swap system starts steering pages to frontswap. Meaning after the backend
has registered it is OK if the pages are still hitting the disk instead of
the backend. Naturally this is unacceptable if one were to unload the
backend (not yet supported) - as we need to be quite atomic at that stage
and need to stop processing the pages the moment the backend is being
unloaded. To support this, the frontswap is using the struct static_key
which are incredibly light when they are in usage. They are incredibly heavy
when the value switches (on/off), but that is OK. The next part of unloading is
also taking the pages that are in the backend and feed them in the swap
storage (and Seth's patches do some of this).

Also attached is one patch from Minchan that fixes the condition where the
backend was constricted in allocating memory at init - b/c we were holding
a spin-lock. His patch fixes that and we are just holding the swapon_mutex
instead. It has been rebased on top of my patches.

This patchset is based on what is going in general for #linux-next for v3.9
(and can be merged alongside Greg's staging tree).

 drivers/staging/zcache/zcache-main.c |  19 +--
 drivers/xen/Kconfig                  |   4 +-
 drivers/xen/tmem.c                   |  53 +++++---
 drivers/xen/xen-selfballoon.c        |  13 +-
 include/linux/cleancache.h           |  27 ++--
 include/linux/frontswap.h            |  31 ++---
 include/xen/tmem.h                   |   8 ++
 mm/cleancache.c                      | 241 ++++++++++++++++++++++++++++++-----
 mm/frontswap.c                       | 126 ++++++++++++++----
 mm/swapfile.c                        |  18 +--
 10 files changed, 417 insertions(+), 123 deletions(-)

Dan Magenheimer (3):
      mm: frontswap: lazy initialization to allow tmem backends to build/run as modules
      mm: cleancache: lazy initialization to allow tmem backends to build/run as modules
      xen: tmem: enable Xen tmem shim to be built/loaded as a module

Konrad Rzeszutek Wilk (7):
      frontswap: Make frontswap_init use a pointer for the ops.
      frontswap: Remove the check for frontswap_enabled.
      frontswap: Use static_key instead of frontswap_enabled and frontswap_ops
      cleancache: Make cleancache_init use a pointer for the ops
      cleancache: Remove the check for cleancache_enabled.
      cleancache: Use static_key instead of cleancache_ops and cleancache_enabled.
      zcache/tmem: Better error checking on frontswap_register_ops return     value.

Minchan Kim (1):
      frontswap: Get rid of swap_lock dependency

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