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Date:   Fri, 11 Nov 2022 09:48:31 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Martin Doucha <mdoucha@suse.cz>
Cc:     Minchan Kim <minchan@kernel.org>, Petr Vorel <pvorel@suse.cz>,
        ltp@lists.linux.it, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        Nitin Gupta <ngupta@vflare.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Jens Axboe <axboe@kernel.dk>,
        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
        Yang Xu <xuyang2018.jy@fujitsu.com>
Subject: Re: [PATCH 0/1] Possible bug in zram on ppc64le on vfat
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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?