mm code for allowing reclaim of page previously swapped but now clean-in-memory?

[Dan Magenheimer <dan.magenheimer@oracle.com>]

I'm looking for mm code/heuristics/flags where the following occurs:

This (anonymous) page was:
- previously swapped to a swap device
- then later read back in from the swap device

Now memory pressure has resulted in a need to reclaim memory so:
- this page is discovered to still be clean, i.e. it
   matches the page still on the swap device, so
- the pageframe is thus an obvious candidate for reclaim

I'd be grateful for any pointers/education...  For example,
is such a page always in the swapcache?  Is it also in the
page cache?  Is it always INactive since it was read but never
written?  What flags are set/unset?  What function or code
snippet identifies such a page and does this code need
to be protected by the swaplock or pagelock or ???
(Sorry if any of these are stupid questions...)

Purpose: I'm looking into zcache (and future KVM/memcg tmem backend)
changes to exploit a "writethrough" and/or "lazy writeback" cacheing
model for pages put into zcache via frontswap, as discussed with Andrea
and one or two others at LSF12/MM.  Either model provides more
flexibility for zcache to more effectively manage persistent pages.

Thanks!
Dan

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