From: Ian Wienand <iwienand@redhat.com>
To: Minchan Kim <minchan@kernel.org>
Cc: 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>,
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: Fri, 4 Aug 2023 16:37:11 +1000 [thread overview]
Message-ID: <ZMycl7xKyJoQNpcu@fedora19.localdomain> (raw)
In-Reply-To: <Y2l3vJb1y2Jynf50@google.com>
On Mon, Nov 07, 2022 at 01:25:16PM -0800, Minchan Kim wrote:
> > 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.
> Is it happening on only ppc64le?
I have managed to replicate this on an arm64 system. I frankly don't
know what is so special about it -- it's a qemu guest and I'm not sure
what exactly it's running ontop of.
> Is it a new regression? What kernel version did you use?
I've replicated this on 4.18.0; obviously something more recent would
be useful but I'm hesitant to destroy too much state in case it is
something ...
> 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.
To recap; this test [1] creates a zram device, makes a filesystem on
it, and fills it with sequential 1k writes from /dev/zero via dd. The
problem is that it sees the mem_used_total for the zram device as zero
in the sysfs stats after the writes; this causes a divide by zero
error in the script calculation.
An annoted extract:
zram01 3 TINFO: /sys/block/zram1/disksize = '26214400'
zram01 3 TPASS: test succeeded
zram01 4 TINFO: set memory limit to zram device(s)
zram01 4 TINFO: /sys/block/zram1/mem_limit = '25M'
zram01 4 TPASS: test succeeded
zram01 5 TINFO: make vfat filesystem on /dev/zram1
>> at this point a cat of /sys/block/zram1/mm_stat shows
>> 65536 527 65536 26214400 65536 0 0 0
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
>> at this point "ls -lh" shows the file
>> total 25M
>> -rwxr-xr-x. 1 root root 25M Aug 4 01:06 file
>> however, /sys/block/zram1/mm_stat shows
>> 9502720 0 0 26214400 196608 145 0 0
>> the script reads this zero value and tries to calculate the
>> compression ratio
./zram01.sh: line 145: 100 * 1024 * 25568 / 0: division by 0 (error token is "0")
>> If we do a "sync" then redisply the mm_stat after, we get
>> 26214400 2842 65536 26214400 196608 399 0 0
I have managed to instrument this, and in the following
static ssize_t mm_stat_show(struct device *dev,
struct device_attribute *attr, char *buf)
{
...
if (init_done(zram)) {
mem_used = zs_get_total_pages(zram->mem_pool);
pr_info("mm_stat_show: init done %p %lld\n", zram->mem_pool, mem_used);
zs_pool_stats(zram->mem_pool, &pool_stats);
zs_get_total_pages(zram->mem_pool) is definitely zero, which is why
the mm_stat is returning zero. i.e. zsmalloc really doesn't seem to
have any pages recorded for that mem_pool ...
This doesn't seem to make sense; how can a device that has a file
system on it not even have one page assigned to it in zram->mem_pool?
I *think* this has something to do with the de-deuplication as noted.
If I stub out page_same_filled() to return false always, we see instead
zram01 7 TPASS: zram1 was filled with '25568' KB
>> < immediately after >
>> 10223616 48516 131072 26214400 196608 0 0 0
>> < after sync >
>> 26214400 126933 327680 26214400 327680 0 0 0
So I think this test still needs a sync to be sure that it's seeing
the right values? It's probably expected that this takes some time to
write everything out?
But is it possible that mem_used_total being zero is a bug -- possibly
triggered by the de-dup path and the test writing the same thing in
every block? Something like the first de-duped page also being thrown
out?
-i
[1] https://github.com/linux-test-project/ltp/blob/8c201e55f684965df2ae5a13ff439b28278dec0d/testcases/kernel/device-drivers/zram/zram01.sh
next prev parent reply other threads:[~2023-08-04 6:38 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-07 19:11 [PATCH 0/1] Possible bug in zram on ppc64le on vfat Petr Vorel
2022-11-07 19:11 ` [PATCH 1/1] zram01.sh: Workaround division by 0 on vfat on ppc64le Petr Vorel
[not found] ` <CAEemH2fYv_=9UWdWB7VDiFOd8EC89qdCbxnPcTPAtGnkwLOYFg@mail.gmail.com>
2022-11-21 8:59 ` [LTP] " Petr Vorel
2022-11-07 21:25 ` [PATCH 0/1] Possible bug in zram on ppc64le on vfat Minchan Kim
2022-11-07 21:47 ` Petr Vorel
2022-11-07 22:42 ` Petr Vorel
2022-11-08 1:05 ` Sergey Senozhatsky
2022-11-09 22:08 ` Petr Vorel
2022-11-10 23:04 ` Minchan Kim
2022-11-11 9:29 ` Petr Vorel
2022-11-10 14:29 ` Martin Doucha
2022-11-11 0:48 ` Sergey Senozhatsky
2022-11-21 9:41 ` Petr Vorel
2022-11-22 14:56 ` Martin Doucha
2022-11-22 15:07 ` Petr Vorel
2022-11-29 4:38 ` Sergey Senozhatsky
2023-05-02 15:23 ` Martin Doucha
2022-11-11 9:18 ` Petr Vorel
2023-08-04 6:37 ` Ian Wienand [this message]
2023-08-07 4:44 ` Ian Wienand
2023-08-07 5:19 ` Sergey Senozhatsky
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