[PATCH part2 v6 0/3] staging: zcache: Support zero-filled pages more efficiently

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

Changelog:
 v5 -> v6:
  * shove variables in debug.c and in debug.h just have an extern, spotted by Konrad
  * update patch description, spotted by Konrad
 v4 -> v5:
  * fix compile error, reported by Fengguang, Geert
  * add check for !is_ephemeral(pool), spotted by Bob
 v3 -> v4:
  * handle duplication in page_is_zero_filled, spotted by Bob
  * fix zcache writeback in dubugfs
  * fix pers_pageframes|_max isn't exported in debugfs
  * fix static variable defined in debug.h but used in multiple C files
  * rebase on Greg's staging-next
 v2 -> v3:
  * increment/decrement zcache_[eph|pers]_zpages for zero-filled pages, spotted by Dan
  * replace "zero" or "zero page" by "zero_filled_page", spotted by Dan
 v1 -> v2:
  * avoid changing tmem.[ch] entirely, spotted by Dan.
  * don't accumulate [eph|pers]pageframe and [eph|pers]zpages for
    zero-filled pages, spotted by Dan
  * cleanup TODO list
  * add Dan Acked-by.

Motivation:

- Seth Jennings points out compress zero-filled pages with LZO(a lossless
  data compression algorithm) will waste memory and result in fragmentation.
  https://lkml.org/lkml/2012/8/14/347
- Dan Magenheimer add "Support zero-filled pages more efficiently" feature
  in zcache TODO list https://lkml.org/lkml/2013/2/13/503

Design:

- For store page, capture zero-filled pages(evicted clean page cache pages and
  swap pages), but don't compress them, set pampd which store zpage address to
  0x2(since 0x0 and 0x1 has already been ocuppied) to mark special zero-filled
  case and take advantage of tmem infrastructure to transform handle-tuple(pool
  id, object id, and an index) to a pampd. Twice compress zero-filled pages will
  contribute to one zcache_[eph|pers]_pageframes count accumulated.
- For load page, traverse tmem hierachical to transform handle-tuple to pampd
  and identify zero-filled case by pampd equal to 0x2 when filesystem reads
  file pages or a page needs to be swapped in, then refill the page to zero
  and return.

Test:

dd if=/dev/zero of=zerofile bs=1MB count=500
vmtouch -t zerofile
vmtouch -e zerofile

formula:
- fragmentation level = (zcache_[eph|pers]_pageframes * PAGE_SIZE - zcache_[eph|pers]_zbytes)
  * 100 / (zcache_[eph|pers]_pageframes * PAGE_SIZE)
- memory zcache occupy = zcache_[eph|pers]_zbytes

Result:

without zero-filled awareness:
- fragmentation level: 98%
- memory zcache occupy: 238MB
with zero-filled awareness:
- fragmentation level: 0%
- memory zcache occupy: 0MB

Wanpeng Li (3):
  staging: zcache: fix static variables defined in debug.h but used in
    mutiple C files
  staging: zcache: introduce zero-filled page stat count
  staging: zcache: clean TODO list

 drivers/staging/zcache/TODO          |    3 +-
 drivers/staging/zcache/debug.c       |   35 +++++++++++++++
 drivers/staging/zcache/debug.h       |   79 ++++++++++++++++++++-------------
 drivers/staging/zcache/zcache-main.c |    4 ++
 4 files changed, 88 insertions(+), 33 deletions(-)

-- 
1.7.5.4

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