** Description changed: + SRU Justification: + ================== + + [Impact] + + * On s390x systems with a small memory footprint, but large amounts of + DASD disks, + + * the memory can get depleted (even during installation) which can + eventually lead to a situation where the OOM kicks in (followed by even + more problems). + + * Starting with kernel 4.18 the patch below leads to 90% memory + consumption savings per active DASD device. + + * The below backport is needed to fix this and get the improvement into + bionic's kernel 4.15. + + [Fix] + + * 3284da34a87ab7a527a593f89bbdaf6debe9e713 3284da3 "s390/dasd: reduce + the default queue depth and nr of hardware queues" + + * Backport: + https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1852257/+attachment/5304830/+files/0001-s390 + -dasd-reduce-the-default-queue-depth-and-nr-of-h.patch + + [Test Case] + + * Configure a s390x system (z/VM guest or LPAR) with only a bit RAM, but + lot's of DASDs devices. + + * Now gradually enable more and more DASDs and monitor the memory usage + (be sure to exclude shared memory and cache). + + * One can notice a difference in mem usage of about 10:1 per activated + DASD comparing the current stock 4.15 kernel with a patches kernel 4.15. + + * With about less than 1GB memory and 40+ DASD devices one may start to + run into an OOM situation w(o the patch. + + [Regression Potential] + + * The regression potential can be considered as moderate, since: + + * this is purely s390x specific + + * it again only affects DASD disk storage (no zFCP/SCSI disk storage) + + * and it's again only limited to smaller systems (so more z/VM guests + rather than LPARs). + + [Other Info] + + * A cherry-pick to 4.15 wasn't clean (problem in one line), hence the + backport, which applied, compiled and worked fine. + + __________ + This fix is already available with Ubuntu 18.10, but need also be integrated into Ubuntu 18.04 LTS kernel (4.15). - This is available git-commit: + This is available git-commit: https://github.com/torvalds/linux/commit/3284da34a87a Backport information will be provided.
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