It would be wrong to ignore the unuse_pte() case: what it's intending
to do is correct, it's just being prevented by the swapcache issue
from doing what it intends at present.
(Though I'm not thrilled with the idea of it causing an admin's
swapoff to fail because of a cgroup reaching mem limit there, I do
agree with your earlier argument that that's the right thing to happen,
and it's up to the admin to fix things up - my original objection came
from not realizing that normally the cgroup will reclaim from itself
to free its mem. Hmm, would the charge fail or the mm get OOM'ed?)
Ignoring add_to/remove_from swap cache is what I've tried before,
and again today. It's not enough: if you trying run a memhog
(something that allocates and touches more memory than the cgroup
is allowed, relying on pushing out to swap to complete), then that
works well with the present accounting in add_to/remove_from swap
cache, but it OOMs once I remove the memcontrol mods from
mm/swap_state.c. I keep going back to investigate why, keep on
thinking I understand it, then later realize I don't. Please
give it a try, I hope you've got better mental models than I have.
And I don't think it will be enough to handle shmem/tmpfs either;
but won't worry about that until we've properly understood why
exempting swapcache leads to those OOMs, and fixed that up.
It checks MEM_CGROUP_TYPE_ALL there, yes; but I can't find anything
checking for either MEM_CGROUP_TYPE_MAPPED or MEM_CGROUP_TYPE_CACHED.
(Or is it hidden in one of those preprocesor ## things which frustrate
both my greps and me!?)
I sent a patch to linux-mm last night, to remove that confusion.
Those indeed are strange behaviours (if the swapoff really has
succeeded, rather than lying), I not seen such and don't have an
explanation. tmpfs doesn't add any weirdness there: when there's
no swap, there can be no swap cache. Or is the swapoff still in
progress? While it's busy, we keep /proc/meminfo looking sensible,
but <Alt><SysRq>m can show negative free swap (IIRC).
I'll be interested to hear what your investigation shows.
Hugh
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