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

From: Minchan Kim <minchan@kernel.org>
To: Petr Vorel <pvorel@suse.cz>
Cc: 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>,
	Martin Doucha <mdoucha@suse.cz>,
	Yang Xu <xuyang2018.jy@fujitsu.com>
Subject: Re: [PATCH 0/1] Possible bug in zram on ppc64le on vfat
Date: Mon, 7 Nov 2022 13:25:16 -0800	[thread overview]
Message-ID: <Y2l3vJb1y2Jynf50@google.com> (raw)
In-Reply-To: <20221107191136.18048-1-pvorel@suse.cz>

On Mon, Nov 07, 2022 at 08:11:35PM +0100, Petr Vorel wrote:
> Hi all,
> 
> following bug is trying to workaround an error on ppc64le, where
> zram01.sh LTP test (there is also kernel selftest
> tools/testing/selftests/zram/zram01.sh, but LTP test got further
> updates) has often mem_used_total 0 although zram is already filled.

Hi, Petr,

Is it happening on only ppc64le?

Is it a new regression? What kernel version did you use?

Actually, mem_used_total indicates how many *physical memory* were
currently used to keep original data size.

However, if the test data is repeated pattern of unsigned long
(https://github.com/torvalds/linux/blob/master/drivers/block/zram/zram_drv.c#L210)
zram doesn't allocate the physical memory but just mark the unsigned long's value
in meta area for decompression later.

Not sure you hit the this case.

> 
> Patch tries to repeatedly read /sys/block/zram*/mm_stat for 1 sec,
> waiting for mem_used_total > 0. The question if this is expected and
> should be workarounded or a bug which should be fixed.
> 
> REPRODUCE THE ISSUE
> Quickest way to install only zram tests and their dependencies:
> make autotools && ./configure && for i in testcases/lib/ testcases/kernel/device-drivers/zram/; do cd $i && make -j$(getconf _NPROCESSORS_ONLN) && make install && cd -; done
> 
> Run the test (only on vfat)
> PATH="/opt/ltp/testcases/bin:$PATH" LTP_SINGLE_FS_TYPE=vfat zram01.sh
> 
> Petr Vorel (1):
>   zram01.sh: Workaround division by 0 on vfat on ppc64le
> 
>  .../kernel/device-drivers/zram/zram01.sh      | 27 +++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)
> 
> -- 
> 2.38.0
> 