Re: [syzbot ci] Re: mm/rmap: index MAP_PRIVATE file-backed folios by virt pgoff

[Lorenzo Stoakes (ARM)]

On Fri, Jul 17, 2026 at 11:47:18PM -0700, syzbot ci wrote:
> syzbot ci has tested the following series
>
> [v1] mm/rmap: index MAP_PRIVATE file-backed folios by virt pgoff
> https://lore.kernel.org/all/20260717-b4-scalable-cow-virt-pgoff-v1-0-cf24910ef094@xxxxxxxxxx
> * [PATCH 01/15] mm/vma: introduce VMA virtual page offset field and add helpers
> * [PATCH 02/15] mm: introduce linear_virt_page_index()
> * [PATCH 03/15] mm: abstract vma_address() and introduce vma_anon_address()
> * [PATCH 04/15] mm: update print_bad_page_map() to show virtual page index
> * [PATCH 05/15] mm: introduce and use vma_filebacked_address()
> * [PATCH 06/15] mm: propagate VMA virtual page offset on map, remap, split + merge
> * [PATCH 07/15] mm/rmap: track whether the page VMA mapped walk is anonymous
> * [PATCH 08/15] mm: introduce and use linear_folio_page_index()
> * [PATCH 09/15] mm/rmap: use virt pgoff for MAP_PRIVATE file-backed anon folios
> * [PATCH 10/15] tools/testing/vma: expand VMA merge tests to assert virt pgoff
> * [PATCH 11/15] tools/testing/selftests/mm: test virtual page offset merge behaviour
> * [PATCH 12/15] mm/vma: only permit MAP_PRIVATE /dev/zero to be mapped anonymous
> * [PATCH 13/15] mm/vma: make MAP_PRIVATE-mapped /dev/zero mappings truly anonymous
> * [PATCH 14/15] tools/testing/vma: add test to assert MAP_PRIVATE-/dev/zero is anon
> * [PATCH 15/15] tools/testing/selftests/mm: add MAP_PRIVATE-/dev/zero merge tests
>
> and found the following issue:
> general protection fault in mapping_wrprotect_range_one
>
> Full report is available here:
> https://ci.syzbot.org/series/b5c827e8-78a4-494c-b925-417c34a60e85
>
> ***
>
> general protection fault in mapping_wrprotect_range_one
>
> tree:      mm-new
> URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/akpm/mm.git
> base:      5093dba1014c1d7f7e247fd118f0fa8f22136046
> arch:      amd64
> compiler:  Debian clang version 22.1.6 (++20260514074242+fc4aad7b5db3-1~exp1~20260514074407.73), Debian LLD 22.1.6
> config:    https://ci.syzbot.org/builds/0d856612-f1b4-4e6e-86af-4ed3215225aa/config
> syz repro: https://ci.syzbot.org/findings/3484dc14-c12f-477e-b14a-6b249c47b2fd/syz_repro
>
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] SMP KASAN PTI
> KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
> CPU: 1 UID: 0 PID: 32 Comm: kworker/1:1 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> Workqueue: events fb_deferred_io_work
> RIP: 0010:folio_test_anon include/linux/page-flags.h:718 [inline]
> RIP: 0010:mapping_wrprotect_range_one+0x147/0x230 mm/rmap.c:1246

Ah yeah, this is:

	VM_WARN_ON_ONCE(folio_test_anon(folio));

In mapping_wrprotect_range_one() and... this test is completely superfluous
actually :)

The only caller is mapping_wrprotect_range() which literally passes a NULL folio
NULL to __rmap_walk_file() (with mapping_wrprotect_range_one set as the
rwc->rmap_one).

I'll ping on the relevant commit and ask Andrew to just drop this line.

Cheers, Lorenzo

> Code: 7c 24 40 49 8d 7f 28 ba 18 00 00 00 31 f6 e8 e0 2e 14 00 48 c7 84 24 80 00 00 00 01 00 00 00 49 83 c6 18 4c 89 f0 48 c1 e8 03 <42> 80 3c 20 00 74 08 4c 89 f7 e8 ea 2b 14 00 4d 8b 36 44 89 f6 83
> RSP: 0018:ffffc90000a777c0 EFLAGS: 00010206
> RAX: 0000000000000003 RBX: ffffc90000a779e0 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffc90000a77840
> RBP: ffffc90000a778e0 R08: ffffc90000a7783f R09: 0000000000000000
> R10: ffffc90000a77828 R11: fffff5200014ef08 R12: dffffc0000000000
> R13: 1ffff9200014eefc R14: 0000000000000018 R15: ffffc90000a77800
> FS:  0000000000000000(0000) GS:ffff8882a920c000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f84a91e9c38 CR3: 000000000e746000 CR4: 00000000000006f0
> Call Trace:
>  <TASK>
>  __rmap_walk_file+0x4cd/0x6a0 mm/rmap.c:3116
>  mapping_wrprotect_range+0x110/0x180 mm/rmap.c:1294
>  fb_deferred_io_work+0x15a/0x310 drivers/video/fbdev/core/fb_defio.c:399
>  process_one_work kernel/workqueue.c:3322 [inline]
>  process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
>  worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
>  kthread+0x388/0x470 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:folio_test_anon include/linux/page-flags.h:718 [inline]
> RIP: 0010:mapping_wrprotect_range_one+0x147/0x230 mm/rmap.c:1246
> Code: 7c 24 40 49 8d 7f 28 ba 18 00 00 00 31 f6 e8 e0 2e 14 00 48 c7 84 24 80 00 00 00 01 00 00 00 49 83 c6 18 4c 89 f0 48 c1 e8 03 <42> 80 3c 20 00 74 08 4c 89 f7 e8 ea 2b 14 00 4d 8b 36 44 89 f6 83
> RSP: 0018:ffffc90000a777c0 EFLAGS: 00010206
> RAX: 0000000000000003 RBX: ffffc90000a779e0 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffc90000a77840
> RBP: ffffc90000a778e0 R08: ffffc90000a7783f R09: 0000000000000000
> R10: ffffc90000a77828 R11: fffff5200014ef08 R12: dffffc0000000000
> R13: 1ffff9200014eefc R14: 0000000000000018 R15: ffffc90000a77800
> FS:  0000000000000000(0000) GS:ffff8882a920c000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000025d35e991000 CR3: 0000000110c34000 CR4: 00000000000006f0
> ----------------
> Code disassembly (best guess):
>    0:	7c 24                	jl     0x26
>    2:	40                   	rex
>    3:	49 8d 7f 28          	lea    0x28(%r15),%rdi
>    7:	ba 18 00 00 00       	mov    $0x18,%edx
>    c:	31 f6                	xor    %esi,%esi
>    e:	e8 e0 2e 14 00       	call   0x142ef3
>   13:	48 c7 84 24 80 00 00 	movq   $0x1,0x80(%rsp)
>   1a:	00 01 00 00 00
>   1f:	49 83 c6 18          	add    $0x18,%r14
>   23:	4c 89 f0             	mov    %r14,%rax
>   26:	48 c1 e8 03          	shr    $0x3,%rax
> * 2a:	42 80 3c 20 00       	cmpb   $0x0,(%rax,%r12,1) <-- trapping instruction
>   2f:	74 08                	je     0x39
>   31:	4c 89 f7             	mov    %r14,%rdi
>   34:	e8 ea 2b 14 00       	call   0x142c23
>   39:	4d 8b 36             	mov    (%r14),%r14
>   3c:	44 89 f6             	mov    %r14d,%esi
>   3f:	83                   	.byte 0x83
>
>
> ***
>
> If these findings have caused you to resend the series or submit a
> separate fix, please add the following tag to your commit message:
>   Tested-by: syzbot@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ---
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