[release-1.0] Cherry-pick #3476: leave page cache out of the memory guard's overshoot check - #3513

…strate#3476)

Follow-up to agent-substrate#3429, from @aramase's review:
agent-substrate#3429 (comment)

When the pod can lend a microVM snapshot write all the memory it needs,
the actor's cgroup has no `memory.high` during the write, so it can end
with a lot of dirty page cache. Putting `memory.high` back reclaims what
it can, but gives up on pages still dirty or under writeback:
https://github.com/torvalds/linux/blob/v7.0/mm/memcontrol.c#L4409-L4429

The memory guard resumes as soon as the write returns, before teardown,
and its overshoot check counted that page cache. So it could kill the
paused VM and report the actor crashed for exceeding its memory limit.

The overshoot check now leaves reclaimable page cache out too, and both
checks take what's left from one `memory.stat` read. The kernel only
flushes `memory.stat` past a threshold of updates, so it can trail
`memory.current`, and subtracting one from the other could count fresh
page cache as used:
https://github.com/torvalds/linux/blob/v7.0/mm/memcontrol.c#L561-L564

AI use: AI-assisted, I reviewed and tested it.

- [x] Tests pass
- [x] Docs updated

---------

Signed-off-by: Benjamin Elder <[email protected]>
(cherry picked from commit 6ddb193)
Signed-off-by: Benjamin Elder <[email protected]>