Seem to lose this from mail bomb. Ping. On Wed, Aug 21, 2013 at 03:16:26PM +0900, Minchan Kim wrote: > 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. > http://lkml.indiana.edu/hypermail/linux/kernel/1304.1/04334.html > > 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. > http://lkml.indiana.edu/hypermail/linux/kernel/1203.3/03812.html > > Second, as more practical solution, we could use device mapper like > dm-cache(https://lwn.net/Articles/540996/), 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 limit > > 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 1G > 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 > > -- > 1.7.9.5 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>