On Thu, Jun 27, 2024 at 10:54:15AM +1000, Alistair Popple wrote: > FS DAX pages have always maintained their own page reference counts > without following the normal rules for page reference counting. 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. > > By treating the refcounts on these pages the same way as normal pages > we can remove a lot of special checks. In particular pXd_trans_huge() > becomes the same as pXd_leaf(), although I haven't made that change > here. It also frees up a valuable SW define PTE bit on architectures > that have devmap PTE bits defined. > > It also almost certainly allows further clean-up of the devmap managed > functions, but I have left that as a future improvment. > > This is an update to the original RFC rebased onto v6.10-rc5. Unlike > the original RFC it passes the same number of ndctl test suite > (https://github.com/pmem/ndctl) tests as my current development > environment does without these patches. I strongly suggest running fstests on pmem devices with '-o dax=always' mount options to get much more comprehensive fsdax test coverage. That exercises a lot of the weird mmap corner cases that cause problems so it would be good to actually test that nothing new got broken in FSDAX by this patchset. -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
