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

[Petr Vorel <pvorel@suse.cz>]

> Hi Minchan,

> > 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?
> I haven't seen it on other archs (x86_64, aarch64).

> > Is it a new regression? What kernel version did you use?
> Found on openSUSE kernel, which uses stable kernel releases 6.0.x.
> It's probably much older, first I've seen it some years ago (I'm not able to find kernel version), but it was random. Now it's much more common.

I tested it on bare metal machine with some older SLES kernel (based on 4.12.14,
with thousands of patches) and it fails:

# PATH="/opt/ltp/testcases/bin:$PATH" LTP_SINGLE_FS_TYPE=vfat zram01.sh
...
zram01 5 TINFO: make vfat filesystem on /dev/zram0
zram01 5 TPASS: zram_makefs succeeded
zram01 6 TINFO: mount /dev/zram0
zram01 6 TPASS: mount of zram device(s) succeeded
zram01 7 TINFO: filling zram0 (it can take long time)
zram01 7 TPASS: zram0 was filled with '25568' KB
/opt/ltp/testcases/bin/zram01.sh: line 137: 100 * 1024 * 25568 / 0: division by 0 (error token is "0")
...

My patch does not help, obviously the value does not change.

zram01 5 TINFO: make vfat filesystem on /dev/zram1
zram01 5 TPASS: zram_makefs succeeded
zram01 6 TINFO: mount /dev/zram1
zram01 6 TPASS: mount of zram device(s) succeeded
zram01 7 TINFO: filling zram1 (it can take long time)
zram01 7 TPASS: zram1 was filled with '25568' KB
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TINFO: /sys/block/zram1/mm_stat
15859712        0        0 26214400   196608      242        0
zram01 7 TBROK: "loop_read_mem_used_total /sys/block/zram1/mm_stat" timed out

This is just to demonstrate, that the problem has been here long time,
and it's not QEMU related (although it could be theoretically related to
openSUSE/SLES user space, but it's very unlikely).

I'll debug if page_same_filled() is being called on mainline kernel.

Kind regards,
Petr

> Test runs on VM (I can give qemu command or whatever you need to know about it)
> I'll try to verify it on some bare metal ppc64le.

> > 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.
> Thanks for a hint, I'll try to debug it.

> Kind regards,
> Petr

> > > 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