On 11. 11. 22 1:48, Sergey Senozhatsky wrote:

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On (22/11/10 15:29), Martin Doucha wrote:

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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

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- mm_stat values stay correct after mount:
65536      220    65536 26214400    65536        0        0        0

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- the bug is triggered by filling the filesystem to capacity (using dd):
4194304        0        0 26214400   327680       64        0        0

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

After 50 test runs on a kernel where the issue is confirmed, I could not
reproduce the failure while filling the device from /dev/urandom instead of
/dev/zero. The test reported compression ratio around 1.8-2.5 which means
the memory usage reported by mm_stat was 10-13MB.

Martin, thanks a lot for reruning tests. I wonder problems on /dev/zero is a
kernel bug or just problem which should be workarounded.

Note that I had to disable the other filesystems in the test because some of
them kept failing with compression ratio <1.

Yes, I noted that as well at least on exfat and btrfs (if I remember correctly).
It wouldn't be a problem to just use it for vfat if we agreed test should be
modified.

Kind regards,
Petr