linux-kernel - Re: [PATCH 0/1] Possible bug in zram on ppc64le on vfat

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Date:   Mon, 21 Nov 2022 10:41:13 +0100
From:   Petr Vorel <pvorel@...e.cz>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Martin Doucha <mdoucha@...e.cz>, Minchan Kim <minchan@...nel.org>,
        ltp@...ts.linux.it, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Nitin Gupta <ngupta@...are.org>, Jens Axboe <axboe@...nel.dk>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Yang Xu <xuyang2018.jy@...itsu.com>
Subject: Re: [PATCH 0/1] Possible bug in zram on ppc64le on vfat

Hi Sergey,

> 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?

I'm not sure if Martin had time to rerun the test. I was not able to reproduce
the problem any more on machine where the test was failing. But I'll have look
into this during this week.

Kind regards,
Petr