a question on mapping from PTE to swap entry

[valdis.kletnieks at vt.edu]

On Thu, 15 Nov 2018 23:53:56 +0800, "Larry" said:

> I'm curious when multiple process has the same PTE which points to the samepage,
> how can kernel differenciate which page from swap space should be swappedin?

The PTE tells where in /dev/swap to find the page.  If it's a shared page, the
references all point at the same place, so "process 1's PAGE_A info" is the
same as "process 2's PAGE_A" info.  They may have different addresses in the
two processes, but they're both referencing the same swap page.

> Process 2 get scheduled, PAGE_A was from Process 1, so it should be swapped out

Why? In general, there's no need to do so - and if it's shared, it's probably a hot page
so it won't be chosen for swapout.

(Remember - the kernel doesn't do swapin/swapout decisions in schedule(), it
only takes action once Process 2 starts running and references a page that requires
something be done.
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