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

[Ian Wienand]

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