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

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Date:   Tue, 22 Nov 2022 16:07:24 +0100
From:   Petr Vorel <pvorel@...e.cz>
To:     Martin Doucha <mdoucha@...e.cz>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        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

> On 11. 11. 22 1:48, Sergey Senozhatsky wrote:
> > On (22/11/10 15:29), Martin Doucha wrote:
> > > 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?

> 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