On Sun, Dec 19, 2021 at 1:48 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> Yes, agreed, I was thinking that we could use "not mapped at all"
> as an optimisation to avoid doing rmap walks. eg __unmap_and_move().
So the thing is, it's a very dodgy optimization for a rather simple
reason: what if somebody pages the page in?
So even "not mapped at all" is questionable.
You have to check that it's also not a swapcache page, and hold the
page lock for that check, at the very least.
And by then, you're really in a very unusual situation - and my gut
feel says not one worth optimizing for (because anon pages are
_usually_ mapped at least once).
But I dunno - it might depend on your load. Maybe you have some very
special load that happens to trigger this case a lot?
Linus
