Hello, (Cc Andrew, Karel)
On (06/24/17 11:08), Sami Kerola wrote: > Hello, > > While going through if there are new util-linux bugs reported I came a > cross this https://bugs.launchpad.net/ubuntu/+source/util-linux/+bug/1645846 > > Simple way to reproduce the issue is: > d=$(cat /sys/class/zram-control/hot_add) && zramctl --size 256M /dev/zram$d > > I am not entirely sure, but drivers/block/zram/zram_drv.c function > zram_add() should block until the device is usable. Looking the code > that it might be the device_add_disk() from block/genhd.c that should > do the blocking. But perhaps it's best if I leave such detail to > people who know the code a bit better. > > One thing annoys me. I expected 'zramctl --find --size 256M' to suffer > from same issue but it does not. I can only reproduce the issue when > triggering hot_add separately, and as quick as possibly using the > path. Notice that sometimes it takes second try before the hot_add and > use triggers the issue. That is almost certainly down to speed the > system in hand, e.g., quicker the computer less likely to trigger. ok... I don't think I see what we can do in zram about the issue in question. what I see on my system is that systemd udevd is racing with zramctl. so, the "failed" case looks like this process action [ 761.191697] zram: >>[systemd-udevd]>>> zram_open ^^^^^^^ systemd opens the device. we have ->bd_openers now [ 761.194057] zram: >>[zramctl]>>> reset_store ^^^^^^^ fails, because device is opened by systemd. if (bdev->bd_openers || zram->claim) is true, we can't reset [ 761.195105] zram: >>[systemd-udevd]>>> zram_release ^^^^^^^ now system releases the device [ 761.198655] zram: >>[zramctl]>>> reset_store ^^^^^^^ succeeds, because we don't have ->bd_openers anymore [ 761.198669] zram: >>[zramctl]>>> zram_reset_device [ 761.198721] zram: >>[zramctl]>>> disksize_store [ 761.199279] zram0: detected capacity change from 0 to 268435456 normal case, process action [ 761.203989] zram: >>[systemd-udevd]>>> zram_open [ 761.204461] zram: >>[systemd-udevd]>>> zram_release [ 761.204940] zram: >>[zramctl]>>> reset_store [ 761.204948] zram: >>[zramctl]>>> zram_reset_device [ 761.204972] zram: >>[zramctl]>>> disksize_store [ 761.205409] zram1: detected capacity change from 0 to 268435456 as you can see, systemd-udevd releases the device before zramctl attempts to reset it. -ss
