On 12/12/2012 09:12 AM, Dan Magenheimer wrote:
From: Dan Magenheimer
Subject: RE: zram /proc/swaps accounting weirdness
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?
I also cannot reproduce it with only zram, without zcache.
I can only reproduce with zcache+zram. Since zcache will
only "fall through" to zram when the frontswap_store() call
in swap_writepage() fails, I wonder if in both cases swap_writepage()
is being called in large (e.g. SWAPFILE_CLUSTER-sized) blocks
of pages? When zram-only, the entire block of pages always gets
sent to zram, but with zcache only a small randomly-positioned
fraction fail frontswap_store(), but the SWAPFILE_CLUSTER-sized
blocks have already been pre-reserved on the swap device and
become only partially-filled?
Urk. Never mind. My bad. When a swap page is compressed in
zcache, it gets accounted in the swap subsystem as an "inuse"
Could you point out to me where add this count to swap subsystem?
page for the backing swap device. (Frontswap provides a
page-by-page "fronting store" for the swap device.) That explains
why Used is so high for the "zram swap device" even though
zram has only compressed a fraction of the pages... the
remaining (much larger) number of pages have been compressed
by/in zcache.
Move along, there are no droids here. :-(
Dan
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