On Wed, Aug 21, 2013 at 06:26:45PM +0800, Bob Liu wrote: > On 08/21/2013 05:24 PM, Bob Liu wrote: > > Hi Minchan, > > > > On 08/21/2013 02:16 PM, 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. > >> > > > > Yes, I didn't notice the feature that zram can be used as a normal block > > device. > > > > > >> 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. > > > > If consider zram as a swap device only, I still think it's better to add > > a pseudo block device to zswap and just disable the writeback of zswap. > > > > But I have no idea of zram's block device feature. > > > > BTW: I think the original/main purpose that zram was introduced is for > swapping. Is there any real users using zram as a normal block device I don't know but when I read http://lwn.net/Articles/334649/, it aimed for compressing page caches as well as swap pages but it made widespread hooks in core (I guess that's why zcache had a birth later by Nitin and Dan) so reviewers guided him to support anon pages only to merge it. And at that time, it was a specific virtual block device for only supporting swap. AFAIRC, akpm suggested to make it general block device so other party can have a benefit. You can type "zram tmp" in google and will find many article related to use zram as tmp and I have been received some questions/reports from anonymous guys by private mail. And Jorome, Redhat guy, has contributed that part like partial I/O. > instead of swap? > For normal usage, maybe we can extend ramdisk with compression feature. Maybe, but I don't see any advantage. The ramdisk is really simple and there is no part to share zram code. Morever, zram have a potential to extend other features like asynchronous, defragmentation, and multiple compressor. I don't want to make simple ramdisk bloating and complicated so every distro could enable it. Another thing is device name in linux is rather straightforward to understand like "dd if=/dev/zero of=/dev/null" but if we support compression feature of ramdisk, "dd if=/dev/ram0 of=/dev/ram1"? Which is compression ramdisk? How could normal user can identify it? I think there is no benefit. > > -- > Regards, > -Bob > > -- > 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>