[syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    9094662f6707 Merge tag 'ata-6.19-rc2' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1411f77c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a11e0f726bfb6765
dashboard link: https://syzkaller.appspot.com/bug?extid=b165fc2e11771c66d8ba
compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11998b1a580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=128cdb1a580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-9094662f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/5bec9d32a91c/vmlinux-9094662f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/3df82e1a3cec/bzImage-9094662f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+b165fc2e11771c66d8ba@syzkaller.appspotmail.com

 handle_mm_fault+0x3fe/0xad0 mm/memory.c:6580
 do_user_addr_fault+0x60c/0x1370 arch/x86/mm/fault.c:1336
 handle_page_fault arch/x86/mm/fault.c:1476 [inline]
 exc_page_fault+0x64/0xc0 arch/x86/mm/fault.c:1532
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
------------[ cut here ]------------
WARNING: ./include/linux/rmap.h:462 at __folio_rmap_sanity_checks include/linux/rmap.h:462 [inline], CPU#1: syz.0.18/6090
WARNING: ./include/linux/rmap.h:462 at __folio_remove_rmap mm/rmap.c:1663 [inline], CPU#1: syz.0.18/6090
WARNING: ./include/linux/rmap.h:462 at folio_remove_rmap_ptes+0xc27/0xfb0 mm/rmap.c:1779, CPU#1: syz.0.18/6090
Modules linked in:
CPU: 1 UID: 0 PID: 6090 Comm: syz.0.18 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:__folio_rmap_sanity_checks include/linux/rmap.h:462 [inline]
RIP: 0010:__folio_remove_rmap mm/rmap.c:1663 [inline]
RIP: 0010:folio_remove_rmap_ptes+0xc27/0xfb0 mm/rmap.c:1779
Code: 00 e9 49 f4 ff ff e8 a8 35 aa ff e8 c3 55 17 ff e9 98 fc ff ff e8 99 35 aa ff 48 c7 c6 80 b7 9c 8b 4c 89 e7 e8 8a 12 f5 ff 90 <0f> 0b 90 e9 5a f6 ff ff e8 7c 35 aa ff 48 8b 54 24 10 48 b8 00 00
RSP: 0018:ffffc90003f5f260 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffffea0001417f80 RCX: ffffc90003f5f144
RDX: ffff88803368c980 RSI: ffffffff8214b106 RDI: ffff88803368ce04
RBP: 0000000000000001 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: ffff88803368d4b0 R12: ffffea0001417f80
R13: ffff888030c90500 R14: 0000000000000000 R15: ffff888012660660
FS:  00007f98fd3fe6c0(0000) GS:ffff8880d69f5000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f98fd3ddd58 CR3: 000000003661c000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 zap_present_folio_ptes mm/memory.c:1650 [inline]
 zap_present_ptes mm/memory.c:1708 [inline]
 do_zap_pte_range mm/memory.c:1810 [inline]
 zap_pte_range mm/memory.c:1854 [inline]
 zap_pmd_range mm/memory.c:1946 [inline]
 zap_pud_range mm/memory.c:1975 [inline]
 zap_p4d_range mm/memory.c:1996 [inline]
 unmap_page_range+0x1b7d/0x43c0 mm/memory.c:2017
 unmap_single_vma+0x153/0x240 mm/memory.c:2059
 unmap_vmas+0x218/0x470 mm/memory.c:2101
 vms_clear_ptes+0x419/0x790 mm/vma.c:1231
 vms_complete_munmap_vmas+0x1ca/0x970 mm/vma.c:1280
 do_vmi_align_munmap+0x446/0x7e0 mm/vma.c:1539
 do_vmi_munmap+0x204/0x3e0 mm/vma.c:1587
 do_munmap+0xb6/0xf0 mm/mmap.c:1065
 mremap_to+0x236/0x450 mm/mremap.c:1378
 remap_move mm/mremap.c:1890 [inline]
 do_mremap+0x13a8/0x2020 mm/mremap.c:1933
 __do_sys_mremap+0x119/0x170 mm/mremap.c:1997
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f98fdd8f7c9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f98fd3fe038 EFLAGS: 00000246 ORIG_RAX: 0000000000000019
RAX: ffffffffffffffda RBX: 00007f98fdfe5fa0 RCX: 00007f98fdd8f7c9
RDX: 0000000000004000 RSI: 0000000000004000 RDI: 0000200000ffc000
RBP: 00007f98fde13f91 R08: 0000200000002000 R09: 0000000000000000
R10: 0000000000000007 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f98fdfe6038 R14: 00007f98fdfe5fa0 R15: 00007ffd69c60518
 </TASK>


---
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Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[David Hildenbrand (Red Hat)]

On 12/23/25 06:23, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    9094662f6707 Merge tag 'ata-6.19-rc2' of git://git.kernel...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1411f77c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a11e0f726bfb6765
> dashboard link: https://syzkaller.appspot.com/bug?extid=b165fc2e11771c66d8ba
> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11998b1a580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=128cdb1a580000
> 
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-9094662f.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/5bec9d32a91c/vmlinux-9094662f.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/3df82e1a3cec/bzImage-9094662f.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b165fc2e11771c66d8ba@syzkaller.appspotmail.com
> 
>   handle_mm_fault+0x3fe/0xad0 mm/memory.c:6580
>   do_user_addr_fault+0x60c/0x1370 arch/x86/mm/fault.c:1336
>   handle_page_fault arch/x86/mm/fault.c:1476 [inline]
>   exc_page_fault+0x64/0xc0 arch/x86/mm/fault.c:1532
>   asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
> ------------[ cut here ]------------
> WARNING: ./include/linux/rmap.h:462 at __folio_rmap_sanity_checks include/linux/rmap.h:462 [inline], CPU#1: syz.0.18/6090

IIUC, that's the

if (folio_test_anon(folio) && !folio_test_ksm(folio)) {
	...
	VM_WARN_ON_FOLIO(atomic_read(&anon_vma->refcount) == 0, folio);
}

Seems to indicate that the anon_vma is no longer alive :/

Fortunately we have a reproducer.

CCing Jann who addded that check "recently".

-- 
Cheers

David

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[Hillf Danton]

On Tue, 23 Dec 2025 09:24:05 +0100 "David Hildenbrand (Red Hat)" wrote:
> On 12/23/25 06:23, syzbot wrote:
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    9094662f6707 Merge tag 'ata-6.19-rc2' of git://git.kernel...
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1411f77c580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=a11e0f726bfb6765
> > dashboard link: https://syzkaller.appspot.com/bug?extid=b165fc2e11771c66d8ba
> > compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11998b1a580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=128cdb1a580000
> > 
> > Downloadable assets:
> > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-9094662f.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/5bec9d32a91c/vmlinux-9094662f.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/3df82e1a3cec/bzImage-9094662f.xz
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+b165fc2e11771c66d8ba@syzkaller.appspotmail.com
> > 
> >   handle_mm_fault+0x3fe/0xad0 mm/memory.c:6580
> >   do_user_addr_fault+0x60c/0x1370 arch/x86/mm/fault.c:1336
> >   handle_page_fault arch/x86/mm/fault.c:1476 [inline]
> >   exc_page_fault+0x64/0xc0 arch/x86/mm/fault.c:1532
> >   asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
> > ------------[ cut here ]------------
> > WARNING: ./include/linux/rmap.h:462 at __folio_rmap_sanity_checks include/linux/rmap.h:462 [inline], CPU#1: syz.0.18/6090
> 
> IIUC, that's the
> 
> if (folio_test_anon(folio) && !folio_test_ksm(folio)) {
> 	...
> 	VM_WARN_ON_FOLIO(atomic_read(&anon_vma->refcount) == 0, folio);
> }
> 
> Seems to indicate that the anon_vma is no longer alive :/
> 
> Fortunately we have a reproducer.
> 
> CCing Jann who addded that check "recently".
>
That check looks incorrect given the atomic_inc_not_zero in folio_get_anon_vma().

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[Harry Yoo]

On Mon, Dec 22, 2025 at 09:23:17PM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    9094662f6707 Merge tag 'ata-6.19-rc2' of git://git.kernel...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1411f77c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a11e0f726bfb6765
> dashboard link: https://syzkaller.appspot.com/bug?extid=b165fc2e11771c66d8ba
> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11998b1a580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=128cdb1a580000
> 
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-9094662f.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/5bec9d32a91c/vmlinux-9094662f.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/3df82e1a3cec/bzImage-9094662f.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b165fc2e11771c66d8ba@syzkaller.appspotmail.com
> 
>  handle_mm_fault+0x3fe/0xad0 mm/memory.c:6580
>  do_user_addr_fault+0x60c/0x1370 arch/x86/mm/fault.c:1336
>  handle_page_fault arch/x86/mm/fault.c:1476 [inline]
>  exc_page_fault+0x64/0xc0 arch/x86/mm/fault.c:1532
>  asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
> ------------[ cut here ]------------
> WARNING: ./include/linux/rmap.h:462 at __folio_rmap_sanity_checks include/linux/rmap.h:462 [inline], CPU#1: syz.0.18/6090
> WARNING: ./include/linux/rmap.h:462 at __folio_remove_rmap mm/rmap.c:1663 [inline], CPU#1: syz.0.18/6090
> WARNING: ./include/linux/rmap.h:462 at folio_remove_rmap_ptes+0xc27/0xfb0 mm/rmap.c:1779, CPU#1: syz.0.18/6090
> Modules linked in:
> CPU: 1 UID: 0 PID: 6090 Comm: syz.0.18 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> RIP: 0010:__folio_rmap_sanity_checks include/linux/rmap.h:462 [inline]
> RIP: 0010:__folio_remove_rmap mm/rmap.c:1663 [inline]
> RIP: 0010:folio_remove_rmap_ptes+0xc27/0xfb0 mm/rmap.c:1779
> Code: 00 e9 49 f4 ff ff e8 a8 35 aa ff e8 c3 55 17 ff e9 98 fc ff ff e8 99 35 aa ff 48 c7 c6 80 b7 9c 8b 4c 89 e7 e8 8a 12 f5 ff 90 <0f> 0b 90 e9 5a f6 ff ff e8 7c 35 aa ff 48 8b 54 24 10 48 b8 00 00
> RSP: 0018:ffffc90003f5f260 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: ffffea0001417f80 RCX: ffffc90003f5f144
> RDX: ffff88803368c980 RSI: ffffffff8214b106 RDI: ffff88803368ce04
> RBP: 0000000000000001 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000001 R11: ffff88803368d4b0 R12: ffffea0001417f80
> R13: ffff888030c90500 R14: 0000000000000000 R15: ffff888012660660
> FS:  00007f98fd3fe6c0(0000) GS:ffff8880d69f5000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f98fd3ddd58 CR3: 000000003661c000 CR4: 0000000000352ef0
> Call Trace:
>  <TASK>
>  zap_present_folio_ptes mm/memory.c:1650 [inline]
>  zap_present_ptes mm/memory.c:1708 [inline]
>  do_zap_pte_range mm/memory.c:1810 [inline]
>  zap_pte_range mm/memory.c:1854 [inline]
>  zap_pmd_range mm/memory.c:1946 [inline]
>  zap_pud_range mm/memory.c:1975 [inline]
>  zap_p4d_range mm/memory.c:1996 [inline]
>  unmap_page_range+0x1b7d/0x43c0 mm/memory.c:2017
>  unmap_single_vma+0x153/0x240 mm/memory.c:2059
>  unmap_vmas+0x218/0x470 mm/memory.c:2101

So this is unmapping VMAs, and it observed an anon_vma with refcount == 0.
anon_vma's refcount isn't supposed to be zero as long as there's
any anonymous memory mapped to a VMA (that's associated with the anon_vma).

From the page dump below, we know that it's been allocated to a file VMA
that has anon_vma (due to CoW, I think).

> [   64.399049][ T6090] page: refcount:2 mapcount:1 mapping:0000000000000000 index:0x0 pfn:0x505fe
> [   64.402037][ T6090] memcg:ffff888100078d40
> [   64.403522][ T6090] anon flags: 0xfff0800002090c(referenced|uptodate|active|owner_2|swapbacked|node=0|zone=1|lastcpupid=0x7ff)
> [   64.407140][ T6090] raw: 00fff0800002090c 0000000000000000 dead000000000122 ffff888012660661
> [   64.409851][ T6090] raw: 0000000000000000 0000000000000000 0000000200000000 ffff888100078d40
> [   64.412578][ T6090] page dumped because: VM_WARN_ON_FOLIO(atomic_read(&anon_vma->refcount) == 0)
> [   64.415320][ T6090] page_owner tracks the page as allocated
> [   64.417353][ T6090] page last allocated via order 0, migratetype Movable, gfp_mask 0x140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), pid 6091, tgid 6089 (syz.0.18), ts 64395709171, free_ts 64007663612
> [   64.422891][ T6090]  post_alloc_hook+0x1af/0x220
> [   64.424399][ T6090]  get_page_from_freelist+0xd0b/0x31a0
> [   64.426135][ T6090]  __alloc_frozen_pages_noprof+0x25f/0x2430
> [   64.427958][ T6090]  alloc_pages_mpol+0x1fb/0x550
> [   64.429506][ T6090]  folio_alloc_mpol_noprof+0x36/0x2f0
> [   64.431157][ T6090]  vma_alloc_folio_noprof+0xed/0x1e0
> [   64.433173][ T6090]  do_fault+0x219/0x1ad0
> [   64.434586][ T6090]  __handle_mm_fault+0x1919/0x2bb0
> [   64.436396][ T6090]  handle_mm_fault+0x3fe/0xad0
> [   64.437985][ T6090]  __get_user_pages+0x54e/0x3590
> [   64.439679][ T6090]  get_user_pages_remote+0x243/0xab0

woohoo, this is faulted via GUP from another process...

> [   64.441359][ T6090]  uprobe_write+0x22b/0x24f0
> [   64.442887][ T6090]  uprobe_write_opcode+0x99/0x1a0
> [   64.444496][ T6090]  set_swbp+0x112/0x200
> [   64.445793][ T6090]  install_breakpoint+0x14b/0xa20
> [   64.447382][ T6090]  uprobe_mmap+0x512/0x10e0
> [   64.448874][ T6090] page last free pid 6082 tgid 6082 stack trace:
> [   64.450887][ T6090]  free_unref_folios+0xa22/0x1610
> [   64.452536][ T6090]  folios_put_refs+0x4be/0x750
> [   64.454064][ T6090]  folio_batch_move_lru+0x278/0x3a0
> [   64.455714][ T6090]  __folio_batch_add_and_move+0x318/0xc30
> [   64.457810][ T6090]  folio_add_lru_vma+0xb0/0x100
> [   64.459416][ T6090]  do_anonymous_page+0x12cf/0x2190
> [   64.461066][ T6090]  __handle_mm_fault+0x1ecf/0x2bb0
> [   64.462706][ T6090]  handle_mm_fault+0x3fe/0xad0
> [   64.464562][ T6090]  do_user_addr_fault+0x60c/0x1370
> [   64.466676][ T6090]  exc_page_fault+0x64/0xc0
> [   64.468067][ T6090]  asm_exc_page_fault+0x26/0x30
> [   64.469661][ T6090] ------------[ cut here ]------------

BUT unfortunately the report doesn't have any information regarding
_when_ the refcount has been dropped to zero.

Perhaps we want yet another DEBUG_VM feature to record when it's been
dropped to zero and report it in the sanity check, or... imagine harder
how a file VMA that has anon_vma involving CoW / GUP / migration /
reclamation could somehow drop the refcount to zero?

Sounds fun ;)

-- 
Cheers,
Harry / Hyeonggon

>  vms_clear_ptes+0x419/0x790 mm/vma.c:1231
>  vms_complete_munmap_vmas+0x1ca/0x970 mm/vma.c:1280
>  do_vmi_align_munmap+0x446/0x7e0 mm/vma.c:1539
>  do_vmi_munmap+0x204/0x3e0 mm/vma.c:1587
>  do_munmap+0xb6/0xf0 mm/mmap.c:1065
>  mremap_to+0x236/0x450 mm/mremap.c:1378
>  remap_move mm/mremap.c:1890 [inline]
>  do_mremap+0x13a8/0x2020 mm/mremap.c:1933
>  __do_sys_mremap+0x119/0x170 mm/mremap.c:1997
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xcd/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f98fdd8f7c9
> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f98fd3fe038 EFLAGS: 00000246 ORIG_RAX: 0000000000000019
> RAX: ffffffffffffffda RBX: 00007f98fdfe5fa0 RCX: 00007f98fdd8f7c9
> RDX: 0000000000004000 RSI: 0000000000004000 RDI: 0000200000ffc000
> RBP: 00007f98fde13f91 R08: 0000200000002000 R09: 0000000000000000
> R10: 0000000000000007 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f98fdfe6038 R14: 00007f98fdfe5fa0 R15: 00007ffd69c60518
>  </TASK>

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[David Hildenbrand (Red Hat)]

On 12/24/25 06:35, Harry Yoo wrote:
> On Mon, Dec 22, 2025 at 09:23:17PM -0800, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    9094662f6707 Merge tag 'ata-6.19-rc2' of git://git.kernel...
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=1411f77c580000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=a11e0f726bfb6765
>> dashboard link: https://syzkaller.appspot.com/bug?extid=b165fc2e11771c66d8ba
>> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11998b1a580000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=128cdb1a580000
>>
>> Downloadable assets:
>> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-9094662f.raw.xz
>> vmlinux: https://storage.googleapis.com/syzbot-assets/5bec9d32a91c/vmlinux-9094662f.xz
>> kernel image: https://storage.googleapis.com/syzbot-assets/3df82e1a3cec/bzImage-9094662f.xz
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+b165fc2e11771c66d8ba@syzkaller.appspotmail.com
>>
>>   handle_mm_fault+0x3fe/0xad0 mm/memory.c:6580
>>   do_user_addr_fault+0x60c/0x1370 arch/x86/mm/fault.c:1336
>>   handle_page_fault arch/x86/mm/fault.c:1476 [inline]
>>   exc_page_fault+0x64/0xc0 arch/x86/mm/fault.c:1532
>>   asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
>> ------------[ cut here ]------------
>> WARNING: ./include/linux/rmap.h:462 at __folio_rmap_sanity_checks include/linux/rmap.h:462 [inline], CPU#1: syz.0.18/6090
>> WARNING: ./include/linux/rmap.h:462 at __folio_remove_rmap mm/rmap.c:1663 [inline], CPU#1: syz.0.18/6090
>> WARNING: ./include/linux/rmap.h:462 at folio_remove_rmap_ptes+0xc27/0xfb0 mm/rmap.c:1779, CPU#1: syz.0.18/6090
>> Modules linked in:
>> CPU: 1 UID: 0 PID: 6090 Comm: syz.0.18 Not tainted syzkaller #0 PREEMPT(full)
>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
>> RIP: 0010:__folio_rmap_sanity_checks include/linux/rmap.h:462 [inline]
>> RIP: 0010:__folio_remove_rmap mm/rmap.c:1663 [inline]
>> RIP: 0010:folio_remove_rmap_ptes+0xc27/0xfb0 mm/rmap.c:1779
>> Code: 00 e9 49 f4 ff ff e8 a8 35 aa ff e8 c3 55 17 ff e9 98 fc ff ff e8 99 35 aa ff 48 c7 c6 80 b7 9c 8b 4c 89 e7 e8 8a 12 f5 ff 90 <0f> 0b 90 e9 5a f6 ff ff e8 7c 35 aa ff 48 8b 54 24 10 48 b8 00 00
>> RSP: 0018:ffffc90003f5f260 EFLAGS: 00010293
>> RAX: 0000000000000000 RBX: ffffea0001417f80 RCX: ffffc90003f5f144
>> RDX: ffff88803368c980 RSI: ffffffff8214b106 RDI: ffff88803368ce04
>> RBP: 0000000000000001 R08: 0000000000000001 R09: 0000000000000000
>> R10: 0000000000000001 R11: ffff88803368d4b0 R12: ffffea0001417f80
>> R13: ffff888030c90500 R14: 0000000000000000 R15: ffff888012660660
>> FS:  00007f98fd3fe6c0(0000) GS:ffff8880d69f5000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00007f98fd3ddd58 CR3: 000000003661c000 CR4: 0000000000352ef0
>> Call Trace:
>>   <TASK>
>>   zap_present_folio_ptes mm/memory.c:1650 [inline]
>>   zap_present_ptes mm/memory.c:1708 [inline]
>>   do_zap_pte_range mm/memory.c:1810 [inline]
>>   zap_pte_range mm/memory.c:1854 [inline]
>>   zap_pmd_range mm/memory.c:1946 [inline]
>>   zap_pud_range mm/memory.c:1975 [inline]
>>   zap_p4d_range mm/memory.c:1996 [inline]
>>   unmap_page_range+0x1b7d/0x43c0 mm/memory.c:2017
>>   unmap_single_vma+0x153/0x240 mm/memory.c:2059
>>   unmap_vmas+0x218/0x470 mm/memory.c:2101
> 
> So this is unmapping VMAs, and it observed an anon_vma with refcount == 0.
> anon_vma's refcount isn't supposed to be zero as long as there's
> any anonymous memory mapped to a VMA (that's associated with the anon_vma).
> 
>  From the page dump below, we know that it's been allocated to a file VMA
> that has anon_vma (due to CoW, I think).
> 
>> [   64.399049][ T6090] page: refcount:2 mapcount:1 mapping:0000000000000000 index:0x0 pfn:0x505fe
>> [   64.402037][ T6090] memcg:ffff888100078d40
>> [   64.403522][ T6090] anon flags: 0xfff0800002090c(referenced|uptodate|active|owner_2|swapbacked|node=0|zone=1|lastcpupid=0x7ff)
>> [   64.407140][ T6090] raw: 00fff0800002090c 0000000000000000 dead000000000122 ffff888012660661
>> [   64.409851][ T6090] raw: 0000000000000000 0000000000000000 0000000200000000 ffff888100078d40
>> [   64.412578][ T6090] page dumped because: VM_WARN_ON_FOLIO(atomic_read(&anon_vma->refcount) == 0)
>> [   64.415320][ T6090] page_owner tracks the page as allocated
>> [   64.417353][ T6090] page last allocated via order 0, migratetype Movable, gfp_mask 0x140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), pid 6091, tgid 6089 (syz.0.18), ts 64395709171, free_ts 64007663612
>> [   64.422891][ T6090]  post_alloc_hook+0x1af/0x220
>> [   64.424399][ T6090]  get_page_from_freelist+0xd0b/0x31a0
>> [   64.426135][ T6090]  __alloc_frozen_pages_noprof+0x25f/0x2430
>> [   64.427958][ T6090]  alloc_pages_mpol+0x1fb/0x550
>> [   64.429506][ T6090]  folio_alloc_mpol_noprof+0x36/0x2f0
>> [   64.431157][ T6090]  vma_alloc_folio_noprof+0xed/0x1e0
>> [   64.433173][ T6090]  do_fault+0x219/0x1ad0
>> [   64.434586][ T6090]  __handle_mm_fault+0x1919/0x2bb0
>> [   64.436396][ T6090]  handle_mm_fault+0x3fe/0xad0
>> [   64.437985][ T6090]  __get_user_pages+0x54e/0x3590
>> [   64.439679][ T6090]  get_user_pages_remote+0x243/0xab0
> 
> woohoo, this is faulted via GUP from another process...
> 
>> [   64.441359][ T6090]  uprobe_write+0x22b/0x24f0
>> [   64.442887][ T6090]  uprobe_write_opcode+0x99/0x1a0
>> [   64.444496][ T6090]  set_swbp+0x112/0x200
>> [   64.445793][ T6090]  install_breakpoint+0x14b/0xa20
>> [   64.447382][ T6090]  uprobe_mmap+0x512/0x10e0
>> [   64.448874][ T6090] page last free pid 6082 tgid 6082 stack trace:
>> [   64.450887][ T6090]  free_unref_folios+0xa22/0x1610
>> [   64.452536][ T6090]  folios_put_refs+0x4be/0x750
>> [   64.454064][ T6090]  folio_batch_move_lru+0x278/0x3a0
>> [   64.455714][ T6090]  __folio_batch_add_and_move+0x318/0xc30
>> [   64.457810][ T6090]  folio_add_lru_vma+0xb0/0x100
>> [   64.459416][ T6090]  do_anonymous_page+0x12cf/0x2190
>> [   64.461066][ T6090]  __handle_mm_fault+0x1ecf/0x2bb0
>> [   64.462706][ T6090]  handle_mm_fault+0x3fe/0xad0
>> [   64.464562][ T6090]  do_user_addr_fault+0x60c/0x1370
>> [   64.466676][ T6090]  exc_page_fault+0x64/0xc0
>> [   64.468067][ T6090]  asm_exc_page_fault+0x26/0x30
>> [   64.469661][ T6090] ------------[ cut here ]------------
> 
> BUT unfortunately the report doesn't have any information regarding
> _when_ the refcount has been dropped to zero.
> 
> Perhaps we want yet another DEBUG_VM feature to record when it's been
> dropped to zero and report it in the sanity check, or... imagine harder
> how a file VMA that has anon_vma involving CoW / GUP / migration /
> reclamation could somehow drop the refcount to zero?
> 
> Sounds fun ;)
> 

Can we bisect the issue given that we have a reproducer?

This only popped up just now, so I would assume it's actually something 
that went into this release that makes it trigger.

-- 
Cheers

David

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[Harry Yoo]

On Tue, Dec 30, 2025 at 11:02:18PM +0100, David Hildenbrand (Red Hat) wrote:
> On 12/24/25 06:35, Harry Yoo wrote:
> > On Mon, Dec 22, 2025 at 09:23:17PM -0800, syzbot wrote:
> > Perhaps we want yet another DEBUG_VM feature to record when it's been
> > dropped to zero and report it in the sanity check, or... imagine harder
> > how a file VMA that has anon_vma involving CoW / GUP / migration /
> > reclamation could somehow drop the refcount to zero?
> > 
> > Sounds fun ;)
> > 
> 
> Can we bisect the issue given that we have a reproducer?

Unfortunately I could not reproduce the issue with the C reproducer,
even with the provided kernel config. Maybe it's a race condition and
I didn't wait long enough...

> This only popped up just now, so I would assume it's actually something that
> went into this release that makes it trigger.

I was assuming the bug has been there even before the addition of
VM_WARN_ON_ONCE(), as the commit a222439e1e27 ("mm/rmap: add anon_vma
lifetime debug check") says:
> There have been syzkaller reports a few months ago[1][2] of UAF in rmap
> walks that seems to indicate that there can be pages with elevated
> mapcount whose anon_vma has already been freed, but I think we never
> figured out what the cause is; and syzkaller only hit these UAFs when
> memory pressure randomly caused reclaim to rmap-walk the affected pages,
> so it of course didn't manage to create a reproducer.
> 
> Add a VM_WARN_ON_FOLIO() when we add/remove mappings of anonymous folios
> to hopefully catch such issues more reliably.

-- 
Cheers,
Harry / Hyeonggon

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[Jeongjun Park]

Harry Yoo wrote:
> On Tue, Dec 30, 2025 at 11:02:18PM +0100, David Hildenbrand (Red Hat) wrote:
> > On 12/24/25 06:35, Harry Yoo wrote:
> > > On Mon, Dec 22, 2025 at 09:23:17PM -0800, syzbot wrote:
> > > Perhaps we want yet another DEBUG_VM feature to record when it's been
> > > dropped to zero and report it in the sanity check, or... imagine harder
> > > how a file VMA that has anon_vma involving CoW / GUP / migration /
> > > reclamation could somehow drop the refcount to zero?
> > > 
> > > Sounds fun ;)
> > > 
> > 
> > Can we bisect the issue given that we have a reproducer?
> 
> Unfortunately I could not reproduce the issue with the C reproducer,
> even with the provided kernel config. Maybe it's a race condition and
> I didn't wait long enough...
> 
> > This only popped up just now, so I would assume it's actually something that
> > went into this release that makes it trigger.
> 
> I was assuming the bug has been there even before the addition of
> VM_WARN_ON_ONCE(), as the commit a222439e1e27 ("mm/rmap: add anon_vma
> lifetime debug check") says:
> > There have been syzkaller reports a few months ago[1][2] of UAF in rmap
> > walks that seems to indicate that there can be pages with elevated
> > mapcount whose anon_vma has already been freed, but I think we never
> > figured out what the cause is; and syzkaller only hit these UAFs when
> > memory pressure randomly caused reclaim to rmap-walk the affected pages,
> > so it of course didn't manage to create a reproducer.
> > 
> > Add a VM_WARN_ON_FOLIO() when we add/remove mappings of anonymous folios
> > to hopefully catch such issues more reliably.
>

I tested this myself and found that the bug is caused by commit
d23cb648e365 ("mm/mremap: permit mremap() move of multiple VMAs").

This commit doesn't mention anything about MREMAP_DONTUNMAP. Is it really
acceptable for MREMAP_DONTUNMAP, which maintains old_address and aliases
new_address, to use move-only fastpath?

If MREMAP_DONTUNMAP can also use fastpath, I think a sophisticated
refactoring of remap_move is needed to manage anon_vma/rmap lifetimes.
Otherwise, adding simple flag check logic to vrm_move_only() is likely
necessary.

What are your thoughts?

> -- 
> Cheers,
> Harry / Hyeonggon

Regards,
Jeongjun Park

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[Harry Yoo]

On Thu, Jan 01, 2026 at 10:09:06PM +0900, Jeongjun Park wrote:
> Harry Yoo wrote:
> > On Tue, Dec 30, 2025 at 11:02:18PM +0100, David Hildenbrand (Red Hat) wrote:
> > > On 12/24/25 06:35, Harry Yoo wrote:
> > > > On Mon, Dec 22, 2025 at 09:23:17PM -0800, syzbot wrote:
> > > > Perhaps we want yet another DEBUG_VM feature to record when it's been
> > > > dropped to zero and report it in the sanity check, or... imagine harder
> > > > how a file VMA that has anon_vma involving CoW / GUP / migration /
> > > > reclamation could somehow drop the refcount to zero?
> > > > 
> > > > Sounds fun ;)
> > > > 
> > > 
> > > Can we bisect the issue given that we have a reproducer?
> > 
> > Unfortunately I could not reproduce the issue with the C reproducer,
> > even with the provided kernel config. Maybe it's a race condition and
> > I didn't wait long enough...
> > 
> > > This only popped up just now, so I would assume it's actually something that
> > > went into this release that makes it trigger.
> > 
> > I was assuming the bug has been there even before the addition of
> > VM_WARN_ON_ONCE(), as the commit a222439e1e27 ("mm/rmap: add anon_vma
> > lifetime debug check") says:
> > > There have been syzkaller reports a few months ago[1][2] of UAF in rmap
> > > walks that seems to indicate that there can be pages with elevated
> > > mapcount whose anon_vma has already been freed, but I think we never
> > > figured out what the cause is; and syzkaller only hit these UAFs when
> > > memory pressure randomly caused reclaim to rmap-walk the affected pages,
> > > so it of course didn't manage to create a reproducer.
> > > 
> > > Add a VM_WARN_ON_FOLIO() when we add/remove mappings of anonymous folios
> > > to hopefully catch such issues more reliably.
> >

Hi Jeongjun,

> I tested this myself and found that the bug is caused by commit
> d23cb648e365 ("mm/mremap: permit mremap() move of multiple VMAs").

Oh, great. Thanks!

Could you please elaborate how you confirmed the bad commit?

- Did you perform git bisection on it?
- How did you reproduce the bug and how long did it take to reproduce?

> This commit doesn't mention anything about MREMAP_DONTUNMAP. Is it really
> acceptable for MREMAP_DONTUNMAP, which maintains old_address and aliases
> new_address, to use move-only fastpath?
> 
> If MREMAP_DONTUNMAP can also use fastpath, I think a sophisticated
> refactoring of remap_move is needed to manage anon_vma/rmap lifetimes.
> Otherwise, adding simple flag check logic to vrm_move_only() is likely
> necessary.
> 
> What are your thoughts?

It's late at night, so...
let me look at at this tomorrow with a clearer mind :)

Happy new year, by the way!

-- 
Cheers,
Harry / Hyeonggon

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[Jeongjun Park]

Hi Harry,

Harry Yoo <harry.yoo@oracle.com> wrote:
>
> On Thu, Jan 01, 2026 at 10:09:06PM +0900, Jeongjun Park wrote:
> > Harry Yoo wrote:
> > > On Tue, Dec 30, 2025 at 11:02:18PM +0100, David Hildenbrand (Red Hat) wrote:
> > > > On 12/24/25 06:35, Harry Yoo wrote:
> > > > > On Mon, Dec 22, 2025 at 09:23:17PM -0800, syzbot wrote:
> > > > > Perhaps we want yet another DEBUG_VM feature to record when it's been
> > > > > dropped to zero and report it in the sanity check, or... imagine harder
> > > > > how a file VMA that has anon_vma involving CoW / GUP / migration /
> > > > > reclamation could somehow drop the refcount to zero?
> > > > >
> > > > > Sounds fun ;)
> > > > >
> > > >
> > > > Can we bisect the issue given that we have a reproducer?
> > >
> > > Unfortunately I could not reproduce the issue with the C reproducer,
> > > even with the provided kernel config. Maybe it's a race condition and
> > > I didn't wait long enough...
> > >
> > > > This only popped up just now, so I would assume it's actually something that
> > > > went into this release that makes it trigger.
> > >
> > > I was assuming the bug has been there even before the addition of
> > > VM_WARN_ON_ONCE(), as the commit a222439e1e27 ("mm/rmap: add anon_vma
> > > lifetime debug check") says:
> > > > There have been syzkaller reports a few months ago[1][2] of UAF in rmap
> > > > walks that seems to indicate that there can be pages with elevated
> > > > mapcount whose anon_vma has already been freed, but I think we never
> > > > figured out what the cause is; and syzkaller only hit these UAFs when
> > > > memory pressure randomly caused reclaim to rmap-walk the affected pages,
> > > > so it of course didn't manage to create a reproducer.
> > > >
> > > > Add a VM_WARN_ON_FOLIO() when we add/remove mappings of anonymous folios
> > > > to hopefully catch such issues more reliably.
> > >
>
> Hi Jeongjun,
>
> > I tested this myself and found that the bug is caused by commit
> > d23cb648e365 ("mm/mremap: permit mremap() move of multiple VMAs").
>
> Oh, great. Thanks!
>
> Could you please elaborate how you confirmed the bad commit?
>
> - Did you perform git bisection on it?
> - How did you reproduce the bug and how long did it take to reproduce?
>

I tested the mremap-related commits in my local environment, building them
one by one and using syzbot repro.

[1] : https://syzkaller.appspot.com/text?tag=ReproC&x=128cdb1a580000

And for debugging purposes, I added the code from commit a222439e1e27
("mm/rmap: add anon_vma lifetime debug check") and ran the test.

Based on my testing, I found that the WARNING starts from commit
d23cb648e365 ("mm/mremap: permit mremap() move of multiple VMAs"),
which is right after commit 2cf442d74216 ("mm/mremap: clean up mlock
populate behavior") in Lorenzo's mremap-related patch series.

```
[  105.610134][ T9699] page: refcount:2 mapcount:1
mapping:0000000000000000 index:0x0 pfn:0x5abd6
[  105.611881][ T9699] memcg:ffff888051abc100
[  105.612642][ T9699] anon flags:
0x4fff0800002090c(referenced|uptodate|active|owner_2|swapbacked|node=1|zone=1|lastcpupid=0x7ff)
[  105.614724][ T9699] raw: 04fff0800002090c 0000000000000000
dead000000000122 ffff888047525bb1
[  105.616213][ T9699] raw: 0000000000000000 0000000000000000
0000000200000000 ffff888051abc100
[  105.617791][ T9699] page dumped because:
VM_WARN_ON_FOLIO(atomic_read(&anon_vma->refcount) == 0)
[  105.619364][ T9699] page_owner tracks the page as allocated
[  105.620554][ T9699] page last allocated via order 0, migratetype
Movable, gfp_mask 0x140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), pid 9700,
tgid 9698 (test), ts 105608898986, free_ts 104692063083
[  105.623518][ T9699]  post_alloc_hook+0x1be/0x230
[  105.624454][ T9699]  get_page_from_freelist+0x10c0/0x2f80
[  105.625446][ T9699]  __alloc_frozen_pages_noprof+0x256/0x2130
[  105.626504][ T9699]  alloc_pages_mpol+0x1f1/0x550
[  105.627383][ T9699]  folio_alloc_mpol_noprof+0x38/0x2f0
[...]
[  105.651729][ T9699] ------------[ cut here ]------------
[  105.652694][ T9699] WARNING: CPU: 0 PID: 9699 at
./include/linux/rmap.h:472 __folio_rmap_sanity_checks+0x6c3/0x770
[  105.654551][ T9699] Modules linked in:
[  105.655268][ T9699] CPU: 0 UID: 0 PID: 9699 Comm: test Not tainted
6.16.0-rc5-00304-gd23cb648e365-dirty #37 PREEMPT(full)
[  105.657209][ T9699] Hardware name: QEMU Standard PC (i440FX + PIIX,
1996), BIOS 1.15.0-1 04/01/2014
[  105.658803][ T9699] RIP: 0010:__folio_rmap_sanity_checks+0x6c3/0x770
[  105.659959][ T9699] Code: 9a 13 00 e9 9f f9 ff ff 4c 89 e7 e8 77 9a
13 00 e9 87 fc ff ff e8 3d d9 af ff 48 c7 c6 00 b1 3b 8b 48 89 ef e8
7e 78 f6 ff 90 <0f> 0b 90 e9 82 fc ff ff e8 80 9a 13 00 e9 32 fa ff ff
e8 76 9a 13
[  105.663311][ T9699] RSP: 0018:ffffc9000baf7268 EFLAGS: 00010293
[  105.664412][ T9699] RAX: 0000000000000000 RBX: 0000000000000000
RCX: ffffc9000baf714c
[  105.665796][ T9699] RDX: ffff888020668000 RSI: ffffffff82089412
RDI: ffff888020668444
[  105.667181][ T9699] RBP: ffffea00016af580 R08: 0000000000000001
R09: ffffed1005704841
[  105.668591][ T9699] R10: 0000000000000001 R11: 0000000000000001
R12: ffff888047525c50
[  105.669977][ T9699] R13: ffff888047525bb0 R14: 0000000000000000
R15: 0000000000000000
[  105.671389][ T9699] FS:  00007f781689e700(0000)
GS:ffff888098559000(0000) knlGS:0000000000000000
[  105.672968][ T9699] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  105.674147][ T9699] CR2: 00007f781687cfb8 CR3: 00000000467b8000
CR4: 0000000000752ef0
[  105.675535][ T9699] PKRU: 55555554
[  105.676175][ T9699] Call Trace:
[  105.676786][ T9699]  <TASK>
[  105.677329][ T9699]  folio_remove_rmap_ptes+0x31/0x980
[  105.678287][ T9699]  unmap_page_range+0x1b97/0x41a0
[  105.679205][ T9699]  ? __pfx_unmap_page_range+0x10/0x10
[  105.680164][ T9699]  ? uprobe_munmap+0x448/0x5d0
[  105.681045][ T9699]  ? uprobe_munmap+0x479/0x5d0
[  105.681916][ T9699]  unmap_single_vma.constprop.0+0x153/0x230
[  105.682973][ T9699]  unmap_vmas+0x1d6/0x430
[  105.683757][ T9699]  ? __pfx_unmap_vmas+0x10/0x10
[  105.684681][ T9699]  ? __sanitizer_cov_trace_switch+0x54/0x90
[  105.685740][ T9699]  ? mas_update_gap+0x30a/0x4f0
[  105.686616][ T9699]  vms_clear_ptes.part.0+0x368/0x690
[  105.687573][ T9699]  ? __pfx_vms_clear_ptes.part.0+0x10/0x10
[  105.688641][ T9699]  ? __pfx_mas_store_gfp+0x10/0x10
[  105.689553][ T9699]  ? unlink_anon_vmas+0x457/0x890
[  105.690463][ T9699]  vms_complete_munmap_vmas+0x6cf/0xa20
[  105.691488][ T9699]  do_vmi_align_munmap+0x426/0x800
[  105.692429][ T9699]  ? __pfx_do_vmi_align_munmap+0x10/0x10
[  105.693456][ T9699]  ? mas_walk+0x6b7/0x8c0
[  105.694290][ T9699]  do_vmi_munmap+0x1f0/0x3d0
[  105.695128][ T9699]  do_munmap+0xbd/0x100
[  105.695883][ T9699]  ? __pfx_do_munmap+0x10/0x10
[  105.696749][ T9699]  ? mas_walk+0x6b7/0x8c0
[  105.697542][ T9699]  mremap_to+0x242/0x450
[  105.698317][ T9699]  do_mremap+0xff4/0x1fe0
[  105.699114][ T9699]  ? __pfx_do_mremap+0x10/0x10
[  105.699992][ T9699]  __do_sys_mremap+0x119/0x170
[  105.700868][ T9699]  ? __pfx___do_sys_mremap+0x10/0x10
[  105.701821][ T9699]  ? __x64_sys_futex+0x1c5/0x4c0
[  105.702712][ T9699]  ? __x64_sys_futex+0x1ce/0x4c0
[  105.703629][ T9699]  do_syscall_64+0xcb/0xfa0
[  105.704463][ T9699]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  105.705510][ T9699] RIP: 0033:0x7f7816996fc9
[  105.706311][ T9699] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f
44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b
4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 97 8e 0d 00 f7 d8
64 89 01 48
[  105.709665][ T9699] RSP: 002b:00007f781689de98 EFLAGS: 00000297
ORIG_RAX: 0000000000000019
[  105.711123][ T9699] RAX: ffffffffffffffda RBX: 0000000000000000
RCX: 00007f7816996fc9
[  105.712507][ T9699] RDX: 0000000000004000 RSI: 0000000000004000
RDI: 0000200000ffc000
[  105.713894][ T9699] RBP: 00007f781689dec0 R08: 0000200000002000
R09: 0000000000000000
[  105.715319][ T9699] R10: 0000000000000007 R11: 0000000000000297
R12: 00007ffddaf7c6fe
[  105.716718][ T9699] R13: 00007ffddaf7c6ff R14: 00007f781689dfc0
R15: 0000000000022000
[  105.718113][ T9699]  </TASK>
[  105.718674][ T9699] Kernel panic - not syncing: kernel: panic_on_warn set ...
[  105.719943][ T9699] CPU: 0 UID: 0 PID: 9699 Comm: test Not tainted
6.16.0-rc5-00304-gd23cb648e365-dirty #37 PREEMPT(full)
[  105.721866][ T9699] Hardware name: QEMU Standard PC (i440FX + PIIX,
1996), BIOS 1.15.0-1 04/01/2014
[  105.723469][ T9699] Call Trace:
[  105.724047][ T9699]  <TASK>
[  105.724592][ T9699]  dump_stack_lvl+0x3d/0x1b0
[  105.725432][ T9699]  panic+0x6fc/0x7b0
[  105.726145][ T9699]  ? __pfx_panic+0x10/0x10
[  105.726955][ T9699]  ? show_trace_log_lvl+0x278/0x380
[  105.727897][ T9699]  ? check_panic_on_warn+0x1f/0xc0
[  105.728819][ T9699]  ? __folio_rmap_sanity_checks+0x6c3/0x770
[  105.729867][ T9699]  check_panic_on_warn+0xb1/0xc0
[  105.730759][ T9699]  __warn+0xf6/0x3d0
[  105.731473][ T9699]  ? __folio_rmap_sanity_checks+0x6c3/0x770
[  105.732522][ T9699]  report_bug+0x2e1/0x500
[  105.733305][ T9699]  ? __folio_rmap_sanity_checks+0x6c3/0x770
[  105.734354][ T9699]  handle_bug+0x2dd/0x410
[  105.735132][ T9699]  exc_invalid_op+0x35/0x80
[  105.735947][ T9699]  asm_exc_invalid_op+0x1a/0x20
[  105.736819][ T9699] RIP: 0010:__folio_rmap_sanity_checks+0x6c3/0x770
[  105.737962][ T9699] Code: 9a 13 00 e9 9f f9 ff ff 4c 89 e7 e8 77 9a
13 00 e9 87 fc ff ff e8 3d d9 af ff 48 c7 c6 00 b1 3b 8b 48 89 ef e8
7e 78 f6 ff 90 <0f> 0b 90 e9 82 fc ff ff e8 80 9a 13 00 e9 32 fa ff ff
e8 76 9a 13
[  105.741281][ T9699] RSP: 0018:ffffc9000baf7268 EFLAGS: 00010293
[  105.742352][ T9699] RAX: 0000000000000000 RBX: 0000000000000000
RCX: ffffc9000baf714c
[  105.743729][ T9699] RDX: ffff888020668000 RSI: ffffffff82089412
RDI: ffff888020668444
[...]
[  105.790634][ T9699] R13: 00007ffddaf7c6ff R14: 00007f781689dfc0
R15: 0000000000022000
[  105.792031][ T9699]  </TASK>
```

And while I haven't been able to reproduce it again, I did have one
instance where a KASAN UAF was detected quite by accident during testing.
So, I suspect UAF might be a low probability occurrence under certain
race conditions.

```
[  142.257627][ T9758]
==================================================================
[  142.259362][ T9758] BUG: KASAN: slab-use-after-free in
folio_remove_rmap_ptes+0x260/0xfc0
[  142.261082][ T9758] Read of size 4 at addr ffff88802856d920 by task test/9758
[  142.262570][ T9758]
[  142.263096][ T9758] CPU: 1 UID: 0 PID: 9758 Comm: test Not tainted
6.19.0-rc2-00098-gc53f467229a7 #20 PREEMPT(full)
[  142.263119][ T9758] Hardware name: QEMU Standard PC (i440FX + PIIX,
1996), BIOS 1.15.0-1 04/01/2014
[  142.263134][ T9758] Call Trace:
[  142.263141][ T9758]  <TASK>
[  142.263148][ T9758]  dump_stack_lvl+0x116/0x1b0
[  142.263187][ T9758]  print_report+0xca/0x5f0
[  142.263219][ T9758]  ? __phys_addr+0xeb/0x180
[  142.263239][ T9758]  ? folio_remove_rmap_ptes+0x260/0xfc0
[  142.263257][ T9758]  ? folio_remove_rmap_ptes+0x260/0xfc0
[  142.263275][ T9758]  kasan_report+0xca/0x100
[  142.263301][ T9758]  ? folio_remove_rmap_ptes+0x260/0xfc0
[  142.263322][ T9758]  kasan_check_range+0x39/0x1c0
[  142.263340][ T9758]  folio_remove_rmap_ptes+0x260/0xfc0
[  142.263360][ T9758]  unmap_page_range+0x1c70/0x4300
[  142.263403][ T9758]  ? __pfx_unmap_page_range+0x10/0x10
[  142.263428][ T9758]  ? uprobe_munmap+0x440/0x600
[  142.263452][ T9758]  ? uprobe_munmap+0x470/0x600
[  142.263472][ T9758]  unmap_single_vma+0x153/0x230
[  142.263499][ T9758]  unmap_vmas+0x1d6/0x430
[  142.263525][ T9758]  ? __pfx_unmap_vmas+0x10/0x10
[  142.263551][ T9758]  ? __sanitizer_cov_trace_switch+0x54/0x90
[  142.263580][ T9758]  ? mas_update_gap+0x30a/0x4f0
[  142.263620][ T9758]  vms_clear_ptes.part.0+0x362/0x6b0
[  142.263642][ T9758]  ? __pfx_vms_clear_ptes.part.0+0x10/0x10
[  142.263666][ T9758]  ? __pfx_mas_store_gfp+0x10/0x10
[  142.263684][ T9758]  ? unlink_anon_vmas+0x457/0x890
[  142.263705][ T9758]  vms_complete_munmap_vmas+0x6cf/0xa20
[  142.263728][ T9758]  do_vmi_align_munmap+0x430/0x800
[  142.263750][ T9758]  ? __pfx_do_vmi_align_munmap+0x10/0x10
[  142.263783][ T9758]  ? mas_walk+0x6b7/0x8c0
[  142.263812][ T9758]  do_vmi_munmap+0x1f0/0x3d0
[  142.263833][ T9758]  do_munmap+0xb6/0xf0
[  142.263860][ T9758]  ? __pfx_do_munmap+0x10/0x10
[  142.263889][ T9758]  ? mas_walk+0x6b7/0x8c0
[  142.263916][ T9758]  mremap_to+0x242/0x450
[  142.263936][ T9758]  do_mremap+0x12b3/0x2090
[  142.263961][ T9758]  ? __pfx_do_mremap+0x10/0x10
[  142.263987][ T9758]  __do_sys_mremap+0x119/0x170
[  142.264007][ T9758]  ? __pfx___do_sys_mremap+0x10/0x10
[  142.264030][ T9758]  ? __x64_sys_futex+0x1c5/0x4d0
[  142.264060][ T9758]  ? __x64_sys_futex+0x1ce/0x4d0
[  142.264095][ T9758]  do_syscall_64+0xcb/0xf80
[  142.264125][ T9758]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  142.264145][ T9758] RIP: 0033:0x7f5736fa5fc9
[  142.264162][ T9758] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f
44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b
4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 97 8e 0d 00 f7 d8
64 89 01 48
[  142.264180][ T9758] RSP: 002b:00007f5736eace98 EFLAGS: 00000297
ORIG_RAX: 0000000000000019
[  142.264201][ T9758] RAX: ffffffffffffffda RBX: 0000000000000000
RCX: 00007f5736fa5fc9
[  142.264213][ T9758] RDX: 0000000000004000 RSI: 0000000000004000
RDI: 0000200000ffc000
[  142.264236][ T9758] RBP: 00007f5736eacec0 R08: 0000200000002000
R09: 0000000000000000
[  142.264247][ T9758] R10: 0000000000000007 R11: 0000000000000297
R12: 00007fff0d19497e
[  142.264258][ T9758] R13: 00007fff0d19497f R14: 00007f5736eacfc0
R15: 0000000000022000
[  142.264277][ T9758]  </TASK>
[  142.264282][ T9758]
[  142.319909][ T9758] Allocated by task 9759:
[  142.320665][ T9758]  kasan_save_stack+0x24/0x50
[  142.321497][ T9758]  kasan_save_track+0x14/0x30
[  142.322331][ T9758]  __kasan_slab_alloc+0x87/0x90
[  142.323193][ T9758]  kmem_cache_alloc_noprof+0x267/0x790
[  142.324151][ T9758]  __anon_vma_prepare+0x34b/0x610
[  142.325035][ T9758]  __vmf_anon_prepare+0x11f/0x250
[  142.325929][ T9758]  do_fault+0x190/0x1940
[  142.326688][ T9758]  __handle_mm_fault+0x1901/0x2ac0
[  142.327581][ T9758]  handle_mm_fault+0x3f9/0xac0
[  142.328424][ T9758]  __get_user_pages+0x5ac/0x3960
[  142.329301][ T9758]  get_user_pages_remote+0x28a/0xb20
[  142.330236][ T9758]  uprobe_write+0x201/0x21f0
[  142.331052][ T9758]  uprobe_write_opcode+0x99/0x1a0
[  142.331936][ T9758]  set_swbp+0x109/0x210
[  142.332677][ T9758]  install_breakpoint+0x158/0x9c0
[  142.333558][ T9758]  uprobe_mmap+0x5ab/0x1070
[  142.334359][ T9758]  vma_complete+0xa00/0xe70
[  142.335157][ T9758]  __split_vma+0xbbb/0x10f0
[  142.335956][ T9758]  vms_gather_munmap_vmas+0x1c5/0x12e0
[  142.336911][ T9758]  __mmap_region+0x475/0x2a70
[  142.337740][ T9758]  mmap_region+0x1b2/0x3e0
[  142.338525][ T9758]  do_mmap+0xa42/0x11e0
[  142.339270][ T9758]  vm_mmap_pgoff+0x280/0x460
[  142.340090][ T9758]  ksys_mmap_pgoff+0x330/0x5d0
[  142.340938][ T9758]  __x64_sys_mmap+0x127/0x190
[  142.341771][ T9758]  do_syscall_64+0xcb/0xf80
[  142.342578][ T9758]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  142.343615][ T9758]
[  142.344035][ T9758] Freed by task 23:
[  142.344708][ T9758]  kasan_save_stack+0x24/0x50
[  142.345537][ T9758]  kasan_save_track+0x14/0x30
[  142.346372][ T9758]  kasan_save_free_info+0x3b/0x60
[  142.347273][ T9758]  __kasan_slab_free+0x61/0x80
[  142.348121][ T9758]  slab_free_after_rcu_debug+0x109/0x300
[  142.349105][ T9758]  rcu_core+0x7a1/0x1600
[  142.349853][ T9758]  handle_softirqs+0x1d4/0x8e0
[  142.350710][ T9758]  run_ksoftirqd+0x3a/0x60
[  142.351503][ T9758]  smpboot_thread_fn+0x3d4/0xaa0
[  142.352377][ T9758]  kthread+0x3d0/0x780
[  142.353103][ T9758]  ret_from_fork+0x966/0xaf0
[  142.353921][ T9758]  ret_from_fork_asm+0x1a/0x30
[  142.354775][ T9758]
[  142.355195][ T9758] Last potentially related work creation:
[  142.356179][ T9758]  kasan_save_stack+0x24/0x50
[  142.357013][ T9758]  kasan_record_aux_stack+0xa7/0xc0
[  142.357924][ T9758]  kmem_cache_free+0x44f/0x760
[  142.358768][ T9758]  __put_anon_vma+0x114/0x390
[  142.359596][ T9758]  unlink_anon_vmas+0x57f/0x890
[  142.360449][ T9758]  move_vma+0x15e1/0x1970
[  142.361214][ T9758]  mremap_to+0x1c3/0x450
[  142.361966][ T9758]  do_mremap+0x12b3/0x2090
[  142.362753][ T9758]  __do_sys_mremap+0x119/0x170
[  142.363596][ T9758]  do_syscall_64+0xcb/0xf80
[  142.364403][ T9758]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  142.365435][ T9758]
[  142.365858][ T9758] The buggy address belongs to the object at
ffff88802856d880
[  142.365858][ T9758]  which belongs to the cache anon_vma of size 208
[  142.368200][ T9758] The buggy address is located 160 bytes inside of
[  142.368200][ T9758]  freed 208-byte region [ffff88802856d880,
ffff88802856d950)
[  142.370541][ T9758]
[  142.370967][ T9758] The buggy address belongs to the physical page:
[  142.372076][ T9758] page: refcount:0 mapcount:0
mapping:0000000000000000 index:0x0 pfn:0x2856d
[  142.373580][ T9758] memcg:ffff888000180f01
[  142.374324][ T9758] ksm flags:
0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
[  142.375621][ T9758] page_type: f5(slab)
[  142.376325][ T9758] raw: 00fff00000000000 ffff888040416140
ffffea000082c080 dead000000000003
[  142.377805][ T9758] raw: 0000000000000000 00000000800f000f
00000000f5000000 ffff888000180f01
[  142.379284][ T9758] page dumped because: kasan: bad access detected
[  142.380392][ T9758] page_owner tracks the page as allocated
[  142.381378][ T9758] page last allocated via order 0, migratetype
Unmovable, gfp_mask
0x52cc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP), pid 7254,
tgid 7254 (systemd-udevd), ts 49831929003, free_ts 49824984874
[  142.384666][ T9758]  post_alloc_hook+0x1ca/0x240
[  142.385505][ T9758]  get_page_from_freelist+0xdb3/0x2a70
[  142.386464][ T9758]  __alloc_frozen_pages_noprof+0x256/0x20f0
[  142.387499][ T9758]  alloc_pages_mpol+0x1f1/0x550
[  142.388365][ T9758]  new_slab+0x2d0/0x440
[  142.389100][ T9758]  ___slab_alloc+0xdd8/0x1bc0
[  142.389927][ T9758]  __slab_alloc.constprop.0+0x66/0x110
[  142.390882][ T9758]  kmem_cache_alloc_noprof+0x4ba/0x790
[  142.391837][ T9758]  anon_vma_fork+0xe6/0x630
[  142.392638][ T9758]  dup_mmap+0x1285/0x2010
[  142.393408][ T9758]  copy_process+0x3747/0x7450
[  142.394236][ T9758]  kernel_clone+0xea/0x880
[  142.395023][ T9758]  __do_sys_clone+0xce/0x120
[  142.395836][ T9758]  do_syscall_64+0xcb/0xf80
[  142.396646][ T9758]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  142.397680][ T9758] page last free pid 7224 tgid 7224 stack trace:
[  142.398937][ T9758]  __free_frozen_pages+0x83e/0x1130
[  142.399864][ T9758]  inode_doinit_with_dentry+0xb0d/0x11f0
[  142.400856][ T9758]  selinux_d_instantiate+0x27/0x30
[  142.401759][ T9758]  security_d_instantiate+0x142/0x1a0
[  142.402709][ T9758]  d_splice_alias_ops+0x94/0x830
[  142.403588][ T9758]  kernfs_iop_lookup+0x23d/0x2d0
[  142.404463][ T9758]  __lookup_slow+0x251/0x480
[  142.405280][ T9758]  lookup_slow+0x51/0x80
[  142.406032][ T9758]  path_lookupat+0x5fe/0xb80
[  142.406851][ T9758]  filename_lookup+0x213/0x5e0
[  142.407701][ T9758]  vfs_statx+0xf2/0x3d0
[  142.408433][ T9758]  __do_sys_newstat+0x96/0x120
[  142.409273][ T9758]  do_syscall_64+0xcb/0xf80
[  142.410083][ T9758]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  142.411114][ T9758]
[  142.411534][ T9758] Memory state around the buggy address:
[  142.412508][ T9758]  ffff88802856d800: fb fb fb fb fb fb fb fb fc
fc fc fc fc fc fc fc
[  142.413894][ T9758]  ffff88802856d880: fa fb fb fb fb fb fb fb fb
fb fb fb fb fb fb fb
[  142.415277][ T9758] >ffff88802856d900: fb fb fb fb fb fb fb fb fb
fb fc fc fc fc fc fc
[  142.416662][ T9758]                                ^
[  142.417549][ T9758]  ffff88802856d980: fc fc fa fb fb fb fb fb fb
fb fb fb fb fb fb fb
[  142.419228][ T9758]  ffff88802856da00: fb fb fb fb fb fb fb fb fb
fb fb fb fc fc fc fc
[  142.420929][ T9758]
==================================================================
[  142.422724][ T9758] Kernel panic - not syncing: KASAN: panic_on_warn set ...
[  142.424255][ T9758] CPU: 1 UID: 0 PID: 9758 Comm: test Not tainted
6.19.0-rc2-00098-gc53f467229a7 #20 PREEMPT(full)
[  142.426503][ T9758] Hardware name: QEMU Standard PC (i440FX + PIIX,
1996), BIOS 1.15.0-1 04/01/2014
[  142.428429][ T9758] Call Trace:
[  142.429138][ T9758]  <TASK>
[  142.429774][ T9758]  dump_stack_lvl+0x3d/0x1b0
[  142.430774][ T9758]  vpanic+0x679/0x710
[  142.431639][ T9758]  panic+0xc2/0xd0
[  142.432427][ T9758]  ? __pfx_panic+0x10/0x10
[  142.433345][ T9758]  ? folio_remove_rmap_ptes+0x260/0xfc0
[  142.434491][ T9758]  ? check_panic_on_warn+0x1f/0xc0
[  142.435548][ T9758]  ? folio_remove_rmap_ptes+0x260/0xfc0
[  142.436738][ T9758]  check_panic_on_warn+0xb1/0xc0
[  142.437805][ T9758]  ? folio_remove_rmap_ptes+0x260/0xfc0
[  142.438986][ T9758]  end_report+0x107/0x160
[  142.439925][ T9758]  kasan_report+0xd8/0x100
[  142.440902][ T9758]  ? folio_remove_rmap_ptes+0x260/0xfc0
[  142.442082][ T9758]  kasan_check_range+0x39/0x1c0
[  142.443111][ T9758]  folio_remove_rmap_ptes+0x260/0xfc0
[  142.444269][ T9758]  unmap_page_range+0x1c70/0x4300
[  142.445370][ T9758]  ? __pfx_unmap_page_range+0x10/0x10
[  142.446520][ T9758]  ? uprobe_munmap+0x440/0x600
[  142.447558][ T9758]  ? uprobe_munmap+0x470/0x600
[  142.448596][ T9758]  unmap_single_vma+0x153/0x230
[  142.449650][ T9758]  unmap_vmas+0x1d6/0x430
[  142.450594][ T9758]  ? __pfx_unmap_vmas+0x10/0x10
[  142.451647][ T9758]  ? __sanitizer_cov_trace_switch+0x54/0x90
[  142.452911][ T9758]  ? mas_update_gap+0x30a/0x4f0
[  142.453966][ T9758]  vms_clear_ptes.part.0+0x362/0x6b0
[  142.455107][ T9758]  ? __pfx_vms_clear_ptes.part.0+0x10/0x10
[  142.456351][ T9758]  ? __pfx_mas_store_gfp+0x10/0x10
[  142.457450][ T9758]  ? unlink_anon_vmas+0x457/0x890
[  142.458523][ T9758]  vms_complete_munmap_vmas+0x6cf/0xa20
[  142.459715][ T9758]  do_vmi_align_munmap+0x430/0x800
[  142.460817][ T9758]  ? __pfx_do_vmi_align_munmap+0x10/0x10
[  142.462034][ T9758]  ? mas_walk+0x6b7/0x8c0
[  142.462971][ T9758]  do_vmi_munmap+0x1f0/0x3d0
[  142.463973][ T9758]  do_munmap+0xb6/0xf0
[  142.464861][ T9758]  ? __pfx_do_munmap+0x10/0x10
[  142.465903][ T9758]  ? mas_walk+0x6b7/0x8c0
[  142.466847][ T9758]  mremap_to+0x242/0x450
[  142.467765][ T9758]  do_mremap+0x12b3/0x2090
[  142.468727][ T9758]  ? __pfx_do_mremap+0x10/0x10
[  142.469763][ T9758]  __do_sys_mremap+0x119/0x170
[  142.470789][ T9758]  ? __pfx___do_sys_mremap+0x10/0x10
[  142.471926][ T9758]  ? __x64_sys_futex+0x1c5/0x4d0
[  142.472986][ T9758]  ? __x64_sys_futex+0x1ce/0x4d0
[  142.474063][ T9758]  do_syscall_64+0xcb/0xf80
[  142.475050][ T9758]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  142.476312][ T9758] RIP: 0033:0x7f5736fa5fc9
[  142.477261][ T9758] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f
44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b
4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 97 8e 0d 00 f7 d8
64 89 01 48
[  142.481362][ T9758] RSP: 002b:00007f5736eace98 EFLAGS: 00000297
ORIG_RAX: 0000000000000019
[  142.483134][ T9758] RAX: ffffffffffffffda RBX: 0000000000000000
RCX: 00007f5736fa5fc9
[  142.484836][ T9758] RDX: 0000000000004000 RSI: 0000000000004000
RDI: 0000200000ffc000
[  142.486536][ T9758] RBP: 00007f5736eacec0 R08: 0000200000002000
R09: 0000000000000000
[  142.488223][ T9758] R10: 0000000000000007 R11: 0000000000000297
R12: 00007fff0d19497e
[  142.489913][ T9758] R13: 00007fff0d19497f R14: 00007f5736eacfc0
R15: 0000000000022000
[  142.491609][ T9758]  </TASK>
```

Since there are no commits in between these two commits, I am certain
that the bug is introduced by this commit.

> > This commit doesn't mention anything about MREMAP_DONTUNMAP. Is it really
> > acceptable for MREMAP_DONTUNMAP, which maintains old_address and aliases
> > new_address, to use move-only fastpath?
> >
> > If MREMAP_DONTUNMAP can also use fastpath, I think a sophisticated
> > refactoring of remap_move is needed to manage anon_vma/rmap lifetimes.
> > Otherwise, adding simple flag check logic to vrm_move_only() is likely
> > necessary.
> >
> > What are your thoughts?
>
> It's late at night, so...
> let me look at at this tomorrow with a clearer mind :)
>
> Happy new year, by the way!

Happy new year to you too! :)

>
> --
> Cheers,
> Harry / Hyeonggon

Regards,
Jeongjun Park

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[Lorenzo Stoakes]

On Thu, Jan 01, 2026 at 11:30:52PM +0900, Jeongjun Park wrote:
>
> Based on my testing, I found that the WARNING starts from commit
> d23cb648e365 ("mm/mremap: permit mremap() move of multiple VMAs"),
> which is right after commit 2cf442d74216 ("mm/mremap: clean up mlock
> populate behavior") in Lorenzo's mremap-related patch series.

OK let me take a look.

Thanks, Lorenzo

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[Lorenzo Stoakes]

On Thu, Jan 01, 2026 at 10:09:06PM +0900, Jeongjun Park wrote:
> Harry Yoo wrote:
> > On Tue, Dec 30, 2025 at 11:02:18PM +0100, David Hildenbrand (Red Hat) wrote:
> > > On 12/24/25 06:35, Harry Yoo wrote:
> > > > On Mon, Dec 22, 2025 at 09:23:17PM -0800, syzbot wrote:
> > > > Perhaps we want yet another DEBUG_VM feature to record when it's been
> > > > dropped to zero and report it in the sanity check, or... imagine harder
> > > > how a file VMA that has anon_vma involving CoW / GUP / migration /
> > > > reclamation could somehow drop the refcount to zero?
> > > >
> > > > Sounds fun ;)
> > > >
> > >
> > > Can we bisect the issue given that we have a reproducer?
> >
> > Unfortunately I could not reproduce the issue with the C reproducer,
> > even with the provided kernel config. Maybe it's a race condition and
> > I didn't wait long enough...
> >
> > > This only popped up just now, so I would assume it's actually something that
> > > went into this release that makes it trigger.
> >
> > I was assuming the bug has been there even before the addition of
> > VM_WARN_ON_ONCE(), as the commit a222439e1e27 ("mm/rmap: add anon_vma
> > lifetime debug check") says:
> > > There have been syzkaller reports a few months ago[1][2] of UAF in rmap
> > > walks that seems to indicate that there can be pages with elevated
> > > mapcount whose anon_vma has already been freed, but I think we never
> > > figured out what the cause is; and syzkaller only hit these UAFs when
> > > memory pressure randomly caused reclaim to rmap-walk the affected pages,
> > > so it of course didn't manage to create a reproducer.
> > >
> > > Add a VM_WARN_ON_FOLIO() when we add/remove mappings of anonymous folios
> > > to hopefully catch such issues more reliably.
> >
>
> I tested this myself and found that the bug is caused by commit
> d23cb648e365 ("mm/mremap: permit mremap() move of multiple VMAs").
>
> This commit doesn't mention anything about MREMAP_DONTUNMAP. Is it really
> acceptable for MREMAP_DONTUNMAP, which maintains old_address and aliases
> new_address, to use move-only fastpath?

It's not a fast path, it permits multiple VMAs to be moved at once for
convenience (most importantly - to avoid users _having to know_ how the kernel
is going to handle VMA merging esp. in the light of confusing rules around
merging of VMAs that map anonymous memory).

When MREMAP_DONTUMAP is used, it doesn't leave the mapping as-is, it moves all
the page tables, it just leaves the existing VMA where it is.

There should be no problem with doing this. Obviously the fact there's a bug
suggests there _is_ a problem obviously.

This should be no different from individually mremap()'ing each of the VMAs
separately.

>
> If MREMAP_DONTUNMAP can also use fastpath, I think a sophisticated
> refactoring of remap_move is needed to manage anon_vma/rmap lifetimes.

Why exactly?

In dontunmap_complete() we unlink all attached anon_vma's explicitly, assuming
we haven't just merged with the VMA we just moved.

We don't have to do so for file-backed VMAs nor should there be any lifetime
issues because the VMA will fault in from the file on access.


> Otherwise, adding simple flag check logic to vrm_move_only() is likely
> necessary.

I'd say let's figure out the bug and see if there's any necessity for this.

So far I haven't been able to reproduce it locally... :) and it seems you could
only reproduce it once so far?

That makes this something of a pain, seems like a race, the fact the repro uses
BPF is also... not great for nailing this down :)

But I am looking into it.

One possibility is it's relying on a just-so arrangement of VMA's that trigger
some horrible merge corner case, this bit of code:

	/*
	 * anon_vma links of the old vma is no longer needed after its page
	 * table has been moved.
	 */
	if (new_vma != vrm->vma && start == old_start && end == old_end)
		unlink_anon_vmas(vrm->vma);

Makes me wonder if a merge that happens to occur here triggers the
!unlink_anon_vmas() case... but then this really shouldn't be any different from
running mremap() repeatedly for each individual VMA.

>
> What are your thoughts?

As Ash from Alien said - I am collating :)

Happy new year to all... :) Am officially on holiday until Monday but will try
to look into this at least for today/tomorrow.

>
> > --
> > Cheers,
> > Harry / Hyeonggon
>
> Regards,
> Jeongjun Park
>

Cheers, Lorenzo

Re: [syzbot] [mm?] WARNING in folio_remove_rmap_ptes

[David Hildenbrand (Red Hat)]

On 1/1/26 17:32, Lorenzo Stoakes wrote:
> On Thu, Jan 01, 2026 at 11:30:52PM +0900, Jeongjun Park wrote:
>>
>> Based on my testing, I found that the WARNING starts from commit
>> d23cb648e365 ("mm/mremap: permit mremap() move of multiple VMAs"),
>> which is right after commit 2cf442d74216 ("mm/mremap: clean up mlock
>> populate behavior") in Lorenzo's mremap-related patch series.
> 
> OK let me take a look.

Trying to make sense of the reproducer and how bpf comes into play ... I 
assume BPF is only used to install a uprobe.

We seem to create a file0 and register a uprobe on it.

We then mmap() that file with PROT_NONE. We should end up in 
uprobe_mmap() and trigger a COW fault -> allocate an anon_vma.

So likely the bpf magic is only there to allocate an anon_vma for a 
PROT_NONE region.

But it's all a bit confusing ... :)

-- 
Cheers

David