[PATCH 00/13] fs/dax: Fix FS DAX page reference counts

[Dan Williams]

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.

Are you seeing the 'mmap.sh' test fail even without these patches?

I see this with the patches, will try without in the morning.

 EXT4-fs (pmem0): unmounting filesystem 26ea1463-343a-464f-9f16-91cb176dbdc7.
 XFS (pmem0): Mounting V5 Filesystem 554953fd-c9f4-460f-bc37-f43979986b68
 XFS (pmem0): Ending clean mount
 Oops: general protection fault, probably for non-canonical address 0xdead000000000518: 00
T SMP PTI
 CPU: 15 PID: 1295 Comm: mmap Tainted: G           OE    N 6.10.0-rc5+ #261
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS edk2-20240524-3.fc40 05/24/2024
 RIP: 0010:folio_mark_dirty+0x25/0x60
 Code: 90 90 90 90 90 0f 1f 44 00 00 53 48 89 fb e8 22 18 02 00 48 85 c0 74 26 48 89 c7 48
0 02 00 74 05 f0 80 63 02 fd <48> 8b 87 18 01 00 00 48 89 de 5b 48 8b 40 18 e9 77 90 c0 00
 RSP: 0018:ffffb073022f7b08 EFLAGS: 00010246
 RAX: 004ffff800002000 RBX: ffffd0d005000300 RCX: 0400000000000040
 RDX: 0000000000000000 RSI: 00007f4006200000 RDI: dead000000000400
 RBP: 0000000000000000 R08: ffff9a4b04504a30 R09: 000fffffffffffff
 R10: ffffd0d005000300 R11: 0000000000000000 R12: 00007f4006200000
 R13: ffff9a4b7c96c000 R14: ffff9a4b7daba440 R15: ffffb073022f7cb0
 FS:  00007f4046351740(0000) GS:ffff9a4d77780000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007f40461ff000 CR3: 000000027aea6000 CR4: 00000000000006f0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  <TASK>
  ? __die_body.cold+0x19/0x26
  ? die_addr+0x38/0x60
  ? exc_general_protection+0x143/0x420
  ? asm_exc_general_protection+0x22/0x30
  ? folio_mark_dirty+0x25/0x60
  ? folio_mark_dirty+0xe/0x60
  unmap_page_range+0xea5/0x1550
  unmap_vmas+0xf8/0x1e0
  unmap_region.constprop.0+0xd7/0x150
  ? lock_is_held_type+0xd5/0x130
  do_vmi_align_munmap.isra.0+0x3f4/0x580
  ? mas_walk+0x101/0x1b0
  __vm_munmap+0xa6/0x170
  __x64_sys_munmap+0x17/0x20
  do_syscall_64+0x75/0x190
  entry_SYSCALL_64_after_hwframe+0x76/0x7e

$ faddr2line vmlinux folio_mark_dirty+0x25
folio_mark_dirty+0x25/0x58:
folio_mark_dirty at mm/page-writeback.c:2860