[LTP] [PATCH 0/1] Possible bug in zram on ppc64le on vfat

[Sergey Senozhatsky]

On (22/11/10 15:29), Martin Doucha wrote:
> New version of LTP test zram01 found a sysfile issue with zram devices
> mounted using VFAT filesystem. When when all available space is filled, e.g.
> by `dd if=/dev/zero of=/mnt/zram0/file`, the corresponding sysfile
> /sys/block/zram0/mm_stat will report that the compressed data size on the
> device is 0 and total memory usage is also 0. LTP test zram01 uses these
> values to calculate compression ratio, which results in division by zero.
> 
> The issue is specific to PPC64LE architecture and the VFAT filesystem. No
> other tested filesystem has this issue and I could not reproduce it on other
> archs (s390 not tested). The issue appears randomly about every 3 test runs
> on SLE-15SP2 and 15SP3 (kernel 5.3). It appears less frequently on SLE-12SP5
> (kernel 4.12). Other SLE version were not tested with the new test version
> yet. The previous version of the test did not check the VFAT filesystem on
> zram devices.

Whoooaa...

> I've tried to debug the issue and collected some interesting data (all
> values come from zram device with 25M size limit and zstd compression
> algorithm):
> - mm_stat values are correct after mkfs.vfat:
> 65536      220    65536 26214400    65536        0        0        0
> 
> - mm_stat values stay correct after mount:
> 65536      220    65536 26214400    65536        0        0        0
> 
> - the bug is triggered by filling the filesystem to capacity (using dd):
> 4194304        0        0 26214400   327680       64        0        0

Can you try using /dev/urandom for dd, not /dev/zero?
Do you still see zeroes in sysfs output or some random values?