Requiring a read from disk is a "hard page fault". A "soft page fault" is when the kernel can satisfy it just by fiddling some flags (eg. marking a pagecache p

Requiring a read from disk is a "hard page fault". A "soft page fault" is when the kernel can satisfy it just by fiddling some flags (eg. marking a pagecache page as dirty) or creating a copy of an already-in-memory page.