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In > > particular pages are considered free when the refcount hits one rather > > than zero and refcounts are not added when mapping the page. > > > Tracking this requires special PTE bits (PTE_DEVMAP) and a secondary > > mechanism for allowing GUP to hold references on the page (see > > get_dev_pagemap). However there doesn't seem to be any reason why FS > > DAX pages need their own reference counting scheme. > > This is fair. However, for anyone coming in fresh to this situation > maybe some more "how we get here" history helps. That longer story is > here: > > http://lore.kernel.org/all/166579181584.2236710.17813547487183983273.stgit@dwillia2-xfh.jf.intel.com/ This never got merged? :( Jason