Hi Dan,

On Fri, Dec 07, 2012 at 03:57:08PM -0800, Dan Magenheimer wrote:

While playing around with zcache+zram (see separate thread),
I was watching stats with "watch -d".

It appears from the code that /sys/block/num_writes only
increases, never decreases.  In my test, num_writes got up

Never decreasement is natural.

to 1863.  /sys/block/disksize is 104857600.

I have two swap disks, one zram (pri=60), one real (pri=-1),
and as a I watched /proc/swaps, the "Used" field grew rapidly
and reached the Size (102396k) of the zram swap, and then
the second swap disk (a physical disk partition) started being
used.  Then for awhile, the Used field for both swap devices
was changing (up and down).

Can you explain how this could happen if num_writes never
exceeded 1863?  This may be harmless in the case where

Odd.
I tried to reproduce it with zram and real swap device without
zcache but failed. Does the problem happen only if enabling zcache
together? 

the only swap on the system is zram; or may indicate a bug
somewhere?

It looks like num_writes is counting bio's not pages...
which would imply the bio's are potentially quite large
(and I'll guess they are of size SWAPFILE_CLUSTER which is
defined to be 256).  Do large clusters make sense with zram?

Swap_writepage handles a page and zram_make_request doesn't use
pluging mechanism of block I/O. So every request for swap-over-zram
is a bio and a page. So your problem might be a BUG.

Late on a Friday so sorry if I am incomprehensible...

P.S. The corresponding stat for zcache indicates that
it failed 8852 stores, so I would have expected zram
to deal with no more than 8852 compressions.

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Kind regards,
Minchan Kim

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