On Mon, Oct 25, 2021 at 04:37:01PM -0700, Linus Torvalds wrote: > For example, traditionally, on x86, with atomic counting there are > three special situations: negative, 0 and positive. So if you use the > traditional x86 counting atomics (just add/sub/inc/dec, no xadd) then > there are situations where you can get more information about the > result in %eflags if you don't use zero as the initial value, but -1. > > Because then you can do "inc", and if ZF is set, you know you were the > _first_ person to increment it. And when you use "dec", and SF is set > afterwards, you know you are the _last_ person to decrement it. > > That was useful when things like "xadd" weren't available, and cmpxchg > loops are expensive. So we used to have counters where -1 was that > "zero point". Very similar to your "1 is the zero point". > > But was it _logical_? No. It was an implementation trick. I think > we've removed all those cases because it was so subtle and confusing > (but maybe we still have it somewhere - I did not check). We still do it for page->_mapcount; it's biased to -1 so that both "the page has exactly one mapping" and "the page has no mapping" are cheaply checkable conditions. But, as you say, that's invisible to the users. page_mapcount() is return atomic_read(&page->_mapcount) + 1;
