Dave Chinner <david@xxxxxxxxxxxxx> writes: > 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. Thanks Dave, I will do that and report back. I suspect it will turn up something, given Dan was seeing a crash with these patches. - Alistair > -Dave.
