On Sun, Aug 18, 2013 at 04:40:49PM +0800, Bob Liu wrote: > This is used to replace previous zram. > zram users can enable this feature, then a pseudo device will be created > automaticlly after kernel boot. > Just using "mkswp /dev/zram0; swapon /dev/zram0" to use it as a swap disk. > > The size of this pseudeo is controlled by zswap boot parameter > zswap.max_pool_percent. > disksize = (totalram_pages * zswap.max_pool_percent/100)*PAGE_SIZE. This /dev/zram0 will behave nothing like the block device that zram creates. It only allows reads/writes to the first PAGE_SIZE area of the device, for mkswap to work, and then doesn't do anything for all other accesses. I guess if you disabled zswap writeback, then... it would somewhat be the same thing. We do need to disable zswap writeback in this case so that zswap does decompressed a ton of pages into the swapcache for writebacks that will just fail. Since zsmalloc does not yet support the reclaim functionality, zswap writeback is implicitly disabled. But this is really weird conceptually since zswap is a caching layer that uses frontswap. If a frontswap store fails, it will try to send the page to the zram0 device which will fail the write. Then the page will be... put back on the active or inactive list? Also, using the max_pool_percent in calculating the psuedo-device size isn't right. Right now, the code makes the device the max size of the _compressed_ pool, but the underlying swap device size is in _uncompressed_ pages. So you'll never be able to fill zswap sizing the device like this, unless every page is highly incompressible to the point that each compressed page effectively uses a memory pool page, in which case, the user shouldn't be using memory compression. This also means that this hasn't been tested in the zswap pool-is-full case since there is no way, in this code, to hit that case. In the zbud case the expected compression is 2:1 so you could just multiply the compressed pool size by 2 and get a good psuedo-device size. With zsmalloc the expected compression is harder to determine since it can achieve very high effective compression ratios on highly compressible pages. Seth -- 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>