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From: Minchan Kim <minchan <at> kernel.org>
Subject: [PATCH v7 0/5] zram/zsmalloc promotion
Newsgroups: gmane.linux.kernel
Date: Wednesday 21st August 2013 06:16:26 UTC (over 3 years ago)
It's 7th trial of zram/zsmalloc promotion.
I rewrote cover-letter totally based on previous discussion.

The main reason to prevent zram promotion was no review of
zsmalloc part while Jens, block maintainer, already acked
zram part.

At that time, zsmalloc was used for zram, zcache and zswap so
everybody wanted to make it general and at last, Mel reviewed it
when zswap was submitted to merge mainline a few month ago.
Most of review was related to zswap writeback mechanism which
can pageout compressed page in memory into real swap storage
in runtime and the conclusion was that zsmalloc isn't good for
zswap writeback so zswap borrowed zbud allocator from zcache to
replace zsmalloc. The zbud is bad for memory compression ratio(2)
but it's very predictable behavior because we can expect a zpage
includes just two pages as maximum. Other reviews were not major. 

Zcache doesn't use zsmalloc either so zsmalloc's user is only
zram now so this patchset moves it into zsmalloc directory.
Recently, Bob tried to move zsmalloc under mm directory to unify
zram and zswap with adding pseudo block device in zswap(It's
very weired to me) but he was simple ignoring zram's block device
(a.k.a zram-blk) feature and considered only swap usecase of zram,
in turn, it lose zram's good concept.

Mel raised an another issue in v6, "maintainance headache".
He claimed zswap and zram has a similar goal that is to compresss
swap pages so if we promote zram, maintainance headache happens
sometime by diverging implementaion between zswap and zram
so that he want to unify zram and zswap. For it, he want zswap
to implement pseudo block device like Bob did to emulate zram so
zswap can have an advantage of writeback as well as zram's benefit.
But I wonder frontswap-based zswap's writeback is really good
approach for writeback POV. I think that problem isn't only
specific for zswap. If we want to configure multiple swap hierarchy
with various speed device such as RAM, NVRAM, SSD, eMMC, NAS etc,
it would be a general problem. So we should think of more general
approach. At a glance, I can see two approach.

First, VM could be aware of heterogeneous swap configuration
so it could aim for being able to configure cache hierarchy
among swap devices. It may need indirction layer on swap, which
was already talked about that way so VM can migrate a block from 
A to B easily. It will support various configuration with VM's
hints, maybe, in future.

Second, as more practical solution, we could use device mapper like
which makes it very
flexible. Now, it supports various configruation and cache policy
(block size, writeback/writethrough, LRU, MFU although MQ is merged
now) so it would be good fit for our purpose. Even, it can make zram
support writeback. I tested it following as following scenario
in KVM 4 CPU, 1G DRAM with background 800M memory hogger, which is
allocates random data up to 800M.

1) zram swap disk 1G, untar kernel.tgz to tmpfs, build -j 4
   Fail to untar due to shortage of memory space by tmpfs default size

2) zram swap disk 1G, untar kernel.tgz to ext2 on zram-blk, build -j 4
   OOM happens while building the kernel but it untar successfully
   on ext2 based on zram-blk. The reason OOM happend is zram can not find
   free pages from main memory to store swap out pages although empty
   swap space is still enough.

3) dm-cache swap disk 1G, untar kernel.tgz to ext2 on zram-blk, build -j 4
   dmcache consists of zram-meta 10M, zram-cache 1G and real swap storage
   No OOM happens and successfully building done.

Above tests proves zram can support writeback into real swap storage
so that zram-cache can always have a free space. If necessary, we could
add new plugin in dm-cache. I see It's really flexible and well-layered
architecure so zram-blk's concept is good for us and it has lots of
potential to be enhanced by MM/FS/Block developers. 

As other disadvantage of zswap writeback, frontswap's semantic is
synchronous API so zswap should decompress in memory zpage
right before writeback and even, it writes pages one by one,
not a batch. If we extend frontswap API, we would enhance it but
I belive we can do better in device mapper layer which is aware of
block align, bandwidth, mapping table, asynchronous and lots of hints
from the block layer. Nonetheless, if we should merge zram's
functionality to zswap, I think zram should include zswap's
functionaliy(But I hope it will never happen) because old age zram
already has lots of real users rather than new young zswap so it's
more handy to unify them with keeping changelog which is one of
valuable things getting from staging stay for a long time.

The reason zram doesn't support writeback until now is just shortage
of needs. The zram's main customers were embedded people so writeback
into real swap storage is too bad for interactivity and wear-leveling
on low falsh devices. But like above, zram has a potential to support
writeback with other block drivers or more reasonable VM enhance
so I'd like to claim zram's block concept is really good.

Another zram-blk's usecase is following as.
The admin can format /dev/zramX with any FS and mount on it.
It could help small memory system, too. For exmaple, many embedded
system don't have swap so although tmpfs can support swapout,
it's pointless. Then, let's assume temp file growing up until half
of system memory once in a while. We don't want to write it on flash
by wear-leveing issue and response problem so we want to keep in-memory.
But if we use tmpfs, it should evict half of working set to cover them
when the size reach peak. In the case, zram-blk would be good fit, too.

I'd like to enhance zram with more features like compaction to prevent
fragmentation problem but zram developers cannot do it now because Greg,
staging maintainer, doesn't want to add new feature until promotion is
done because zram have been in staging for a very long time. Acutally,
some patches about enhance are pending for a long time.

It's time to promote and let's make further enhancements.

Patch 1 adds new Kconfig for zram to use page table method instead
of copy. Andrew suggested it.

Patch 2 adds lots of comment for zsmalloc.

Patch 3 moves zsmalloc under driver/staging/zram because zram is only
user for zram now.

Patch 4 makes unmap_kernel_range exportable function because zsmalloc
have used map_vm_area which is already exported function so zsmalloc
need to use unmap_kernel_range for building as module.

Patch 5 moves zram from driver/staging to driver/blocks, finally.

It touches mm, staging, blocks so I am not sure who is right position
maintainer so I will Cc Andrew, Jens and Greg.

Minchan Kim (4):
  zsmalloc: add Kconfig for enabling page table method
  zsmalloc: move it under zram
  mm: export unmap_kernel_range
  zram: promote zram from staging

Nitin Cupta (1):
  zsmalloc: add more comment

 drivers/block/Kconfig                    |    2 +
 drivers/block/Makefile                   |    1 +
 drivers/block/zram/Kconfig               |   37 +
 drivers/block/zram/Makefile              |    3 +
 drivers/block/zram/zram.txt              |   71 ++
 drivers/block/zram/zram_drv.c            |  987
 drivers/block/zram/zsmalloc.c            | 1084
 drivers/staging/Kconfig                  |    4 -
 drivers/staging/Makefile                 |    2 -
 drivers/staging/zram/Kconfig             |   25 -
 drivers/staging/zram/Makefile            |    3 -
 drivers/staging/zram/zram.txt            |   77 ---
 drivers/staging/zram/zram_drv.c          |  984
 drivers/staging/zram/zram_drv.h          |  125 ----
 drivers/staging/zsmalloc/Kconfig         |   10 -
 drivers/staging/zsmalloc/Makefile        |    3 -
 drivers/staging/zsmalloc/zsmalloc-main.c | 1063
 drivers/staging/zsmalloc/zsmalloc.h      |   43 --
 include/linux/zram.h                     |  123 ++++
 include/linux/zsmalloc.h                 |   52 ++
 mm/vmalloc.c                             |    1 +
 21 files changed, 2361 insertions(+), 2339 deletions(-)
 create mode 100644 drivers/block/zram/Kconfig
 create mode 100644 drivers/block/zram/Makefile
 create mode 100644 drivers/block/zram/zram.txt
 create mode 100644 drivers/block/zram/zram_drv.c
 create mode 100644 drivers/block/zram/zsmalloc.c
 delete mode 100644 drivers/staging/zram/Kconfig
 delete mode 100644 drivers/staging/zram/Makefile
 delete mode 100644 drivers/staging/zram/zram.txt
 delete mode 100644 drivers/staging/zram/zram_drv.c
 delete mode 100644 drivers/staging/zram/zram_drv.h
 delete mode 100644 drivers/staging/zsmalloc/Kconfig
 delete mode 100644 drivers/staging/zsmalloc/Makefile
 delete mode 100644 drivers/staging/zsmalloc/zsmalloc-main.c
 delete mode 100644 drivers/staging/zsmalloc/zsmalloc.h
 create mode 100644 include/linux/zram.h
 create mode 100644 include/linux/zsmalloc.h


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