Re: How to fake a dax device for debugging purposes?

(Please keep me on the cc line since I'm not okn the linux-nvdimm list.) Hi, I used to fake up a dax-capable device for debugging ext4 by using instructions similar to the ones that can be found here: https://docs.pmem.io/persistent-memory/getting-started-guide/creating-dev... The problem is that with more recent kernels, this is no longer working for me. Here are the relevant dmesg lines (from running "gce-xfstests -c dax launch"): [ 0.000000] Linux version 5.7.0-rc4-xfstests-00002-g8867a85a3164-dirty (tytso@lambda) (gcc version 9.3.0 (Debian 9.3.0-11), GNU ld (GNU Binutils for Debian) 2.34) #1692 SMP Sun May 10 21:21:14 EDT 2020 [ 0.000000] Command line: root=/dev/sda1 ro console=ttyS0,38400n8 elevator=noop net.ifnames=0 biosdevname=0 console=ttyS0 memmap=4G!9G memmap=9G!14G cmd=maint mem=26624M fstestcfg= fstestset= fstestexc= fstestopt= fstesttyp=ext4 fstestapi=1.5 fsteststr= nfssrv= ... [ 0.000000] user: [mem 0x0000000240000000-0x000000033fffffff] persistent (type 12) [ 0.000000] user: [mem 0x0000000340000000-0x000000037fffffff] usable [ 0.000000] user: [mem 0x0000000380000000-0x00000005bfffffff] persistent (type 12) [ 0.000000] user: [mem 0x00000005c0000000-0x000000067fffffff] usable .... [ 3.180904] nd_pmem namespace0.0: unable to guarantee persistence of writes [ 3.181750] nd_pmem namespace1.0: unable to guarantee persistence of writes [ 3.188025] pmem0: detected capacity change from 0 to 4294967296 [ 3.189896] pmem1: detected capacity change from 0 to 9663676416 But when I try to mount a file system with: "mount -o dax -t ext4 /dev/pmem0 /mnt" I get: [ 168.136331] EXT4-fs (pmem0): DAX unsupported by block device. Looking at drivers/dax/super.c, and changing a bunch of pr_debug to pr_err, I found the following had triggered. [ 168.130603] pmem0: error: request queue doesn't support dax So looks like drivers/nvdimm/pmem.c is failing to set QUEUE_FLAG_DAX flag on its queue, and so in turn that's because pmem->pfn_flags doesn't have PFN_MAP set. And.... at that point, I'm lost. How do I make a /dev/pmem0 via the memmap= boot command line options be mountable as a dax mount file system?