Re: [PATCH V10 0/8] dax: prepare for famfs

[Alison Schofield <alison.schofield@intel.com>]

On Mon, Mar 30, 2026 at 02:21:48PM -0700, Alison Schofield wrote:
> On Fri, Mar 27, 2026 at 09:03:26PM +0000, John Groves wrote:
> > From: John Groves <john@groves.net>
> > 
> > This patch series along with the bundled patches to fuse are available
> > as a git tag at [0].
> > 
> > Dropped the "bundle" thread. If this submission goes smoothly, I'll update
> > the fuse patches to v10 (very little change there as yet).
> > 
> > Changes v9 -> v10
> > - Minor modernizations per comments from (mostly) Jonathan
> > - Minor Kconfig simplification
> > - bus.c:dax_match_type(): don't make fsdev_dax eligible for automatic binding
> >   where devdax would otherwise bind
> > - dax-private.h: add missing kerneldoc comment for field cached_size in
> >   struct dev_dax_range (thanks Dave)
> > - fsdev_write_dax(): s/pmem_addr/addr/ (thanks Dave)
> > - include/linux/dax.h: remove a spuriously-added declaration of inode_dax()
> >   (thanks Jonathan)
> > 
> > Description:
> > 
> > This patch series introduces the required dax support for famfs.
> > Previous versions of the famfs series included both dax and fuse patches.
> > This series separates them into separate patch series' (and the fuse
> > series dependends on this dax series).
> > 
> > The famfs user space code can be found at [1]
> > 
> > Dax Overview:
> > 
> > This series introduces a new "famfs mode" of devdax, whose driver is
> > drivers/dax/fsdev.c. This driver supports dax_iomap_rw() and
> > dax_iomap_fault() calls against a character dax instance. A dax device
> > now can be converted among three modes: 'system-ram', 'devdax' and
> > 'famfs' via daxctl or sysfs (e.g. unbind devdax and bind famfs instead).
> > 
> > In famfs mode, a dax device initializes its pages consistent with the
> > fsdaxmode of pmem. Raw read/write/mmap are not supported in this mode,
> > but famfs is happy in this mode - using dax_iomap_rw() for read/write and
> > dax_iomap_fault() for mmap faults.
> > 
> 
> Here's what I found:
> 
> famfs-v10 on 7.0-rc5 + ndctl v84:
> 	dax suite all pass 13/13, so no regression appears
> 
> famfs-v10 on 7.0-rc5 +
> (ndctl v84 w https://github.com/jagalactic/ndctl/tree/famfs
> top 3 patches + edit daxctl-famfs.sh to use cxl-test:
> 
> 	existing dax suite keeps passing
> 	daxctl-famfs.sh oops w the new test at # Restore original mode"
> 	seems easy to repoduce, maybe cannot go back to system-ram???

My stack trace differed from Ira's.  I hit:

[   88.991865] probe of dax0.0 returned 0 after 2371506 usecs
[   88.996717] page: refcount:0 mapcount:1 mapping:0000000000000000 index:0x0 pfn:0x3ff028000
[   88.997592] BUG: unable to handle page fault for address: ffffc9000f4c8033
[   88.998256] #PF: supervisor read access in kernel mode
[   88.998728] #PF: error_code(0x0000) - not-present page
[   88.999254] PGD 80a067 P4D 80a067 PUD 193e067 PMD 79baf067 PTE 0
[   88.999799] Oops: Oops: 0000 [#1] SMP NOPTI
[   89.000253] CPU: 5 UID: 0 PID: 1476 Comm: daxctl Tainted: G           O        7.0.0-rc5+ #182 PREEMPT(full) 
[   89.001092] Tainted: [O]=OOT_MODULE
[   89.001630] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
[   89.002345] RIP: 0010:is_free_buddy_page+0x39/0x60
[   89.002816] Code: 00 00 00 48 c1 fe 06 eb 0a 48 83 c1 01 48 83 f9 0b 74 30 44 89 c0 48 89 fa d3 e0 83 e8 01 48 98 48 21 f0 48 c1 e0 06 48 29 c2 <80> 7a 33 f0 75 d9 48 8b 42 28 48 39 c8 72 d0 b8 01 00 00 00 e9 ce
[   89.004504] RSP: 0018:ffffc9000f4cf828 EFLAGS: 00010286
[   89.005039] RAX: 0000000000007a80 RBX: ffffc9000f4cf8a0 RCX: 0000000000000009
[   89.005674] RDX: ffffc9000f4c8000 RSI: ffffff7c003d33ea RDI: ffffc9000f4cfa80
[   89.006350] RBP: ffffc9000f4cf838 R08: 0000000000000001 R09: 00000000ffefffff
[   89.007000] R10: ffffc9000f4cfa38 R11: ffff888376ffe000 R12: ffffc9000f4cfa80
[   89.007673] R13: ffffc9000f4cf9a0 R14: 0000000000000006 R15: 0000000000000001
[   89.008395] FS:  00007f3fbca2e7c0(0000) GS:ffff8881fa75f000(0000) knlGS:0000000000000000
[   89.009156] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   89.009715] CR2: ffffc9000f4c8033 CR3: 000000012f638003 CR4: 0000000000370ef0
[   89.010447] Call Trace:
[   89.010767]  <TASK>
[   89.011083]  ? set_ps_flags.constprop.0+0x3c/0x70
[   89.011559]  snapshot_page+0x2ca/0x330
[   89.011974]  __dump_page+0x2e/0x380
[   89.012362]  ? up+0x5a/0x90
[   89.012704]  dump_page+0x16/0x50
[   89.013108]  ? dump_page+0x16/0x50
[   89.013489]  __get_pfnblock_flags_mask+0x6f/0xd0
[   89.013958]  get_pfnblock_migratetype+0xe/0x30
[   89.014412]  __dump_page+0x15b/0x380
[   89.014816]  dump_page+0x16/0x50
[   89.015210]  ? dump_page+0x16/0x50
[   89.015587]  __set_pfnblock_flags_mask.constprop.0+0x6f/0xf0
[   89.016195]  init_pageblock_migratetype+0x39/0x60
[   89.016692]  memmap_init_range+0x165/0x290
[   89.017205]  move_pfn_range_to_zone+0xed/0x200
[   89.017688]  mhp_init_memmap_on_memory+0x23/0xb0
[   89.018223]  memory_subsys_online+0x127/0x1a0
[   89.018693]  device_online+0x4d/0x90
[   89.019149]  state_store+0x96/0xa0
[   89.019552]  dev_attr_store+0x12/0x30
[   89.019975]  sysfs_kf_write+0x48/0x70
[   89.020381]  kernfs_fop_write_iter+0x160/0x210
[   89.020876]  vfs_write+0x261/0x500
[   89.021311]  ksys_write+0x5c/0xf0
[   89.021701]  __x64_sys_write+0x14/0x20
[   89.022180]  x64_sys_call+0x1cb7/0x2010
[   89.022640]  do_syscall_64+0xb1/0x560
[   89.023096]  entry_SYSCALL_64_after_hwframe+0x71/0x79
[   89.023615] RIP: 0033:0x7f3fbc901c37
[   89.024050] Code: 0f 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
[   89.025768] RSP: 002b:00007ffdbdf63c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[   89.026517] RAX: ffffffffffffffda RBX: 00007ffdbdf64228 RCX: 00007f3fbc901c37
[   89.027280] RDX: 000000000000000f RSI: 00007f3fbcb554de RDI: 0000000000000004
[   89.027934] RBP: 00007ffdbdf63ca0 R08: 0000000000000000 R09: 0000000000000073
[   89.028610] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
[   89.029337] R13: 00007ffdbdf64260 R14: 0000000000414da0 R15: 00007f3fbcb9b000
[   89.030051]  </TASK>
[   89.030364] Modules linked in: cxl_test(O) cxl_acpi(O) cxl_pmem(O) device_dax(O) fsdev_dax kmem dax_pmem(O) nd_pmem(O) dax_cxl nd_btt(O) nd_e820(O) nfit(O) cxl_mock_mem(O) cxl_mem(O) cxl_port(O) cxl_mock(O) libnvdimm(O) nfit_test_iomap(O) cxl_core(O) fwctl [last unloaded: cxl_pmem(O)]
[   89.032575] CR2: ffffc9000f4c8033
[   89.032960] ---[ end trace 0000000000000000 ]---
[   89.033460] RIP: 0010:is_free_buddy_page+0x39/0x60
[   89.033948] Code: 00 00 00 48 c1 fe 06 eb 0a 48 83 c1 01 48 83 f9 0b 74 30 44 89 c0 48 89 fa d3 e0 83 e8 01 48 98 48 21 f0 48 c1 e0 06 48 29 c2 <80> 7a 33 f0 75 d9 48 8b 42 28 48 39 c8 72 d0 b8 01 00 00 00 e9 ce
[   89.035645] RSP: 0018:ffffc9000f4cf828 EFLAGS: 00010286
[   89.036235] RAX: 0000000000007a80 RBX: ffffc9000f4cf8a0 RCX: 0000000000000009
[   89.036910] RDX: ffffc9000f4c8000 RSI: ffffff7c003d33ea RDI: ffffc9000f4cfa80
[   89.037588] RBP: ffffc9000f4cf838 R08: 0000000000000001 R09: 00000000ffefffff
[   89.038310] R10: ffffc9000f4cfa38 R11: ffff888376ffe000 R12: ffffc9000f4cfa80
[   89.039008] R13: ffffc9000f4cf9a0 R14: 0000000000000006 R15: 0000000000000001
[   89.039710] FS:  00007f3fbca2e7c0(0000) GS:ffff8881fa75f000(0000) knlGS:0000000000000000
[   89.040506] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   89.041129] CR2: ffffc9000f4c8033 CR3: 000000012f638003 CR4: 0000000000370ef0
[   89.041836] note: daxctl[1476] exited with irqs disabled
 


> 
> Let me know if you need more info.
> 
> -- Alison
> 
>