On Sun, Dec 19, 2021 at 01:53:36PM -0800, Linus Torvalds wrote: > 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). I'd like to get rid of ->mapcount for file pages too. And those are definitely never mapped in the majority of cases.
