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

[Lance Yang <lance.yang@linux.dev>]

+Cc Usama

On Tue, Mar 31, 2026 at 11:08:27PM -0700, syzbot wrote:
>Hello,
>
>syzbot found the following issue on:
>
>HEAD commit:    cf7c3c02fdd0 Add linux-next specific files for 20260330
>git tree:       linux-next
>console output: https://syzkaller.appspot.com/x/log.txt?x=154ee46a580000
>kernel config:  https://syzkaller.appspot.com/x/.config?x=3944d875fa9bfb67
>dashboard link: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085
>compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
>syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12c846ba580000
>
>Downloadable assets:
>disk image: https://storage.googleapis.com/syzbot-assets/053d3b49a360/disk-cf7c3c02.raw.xz
>vmlinux: https://storage.googleapis.com/syzbot-assets/faabb37d41d0/vmlinux-cf7c3c02.xz
>kernel image: https://storage.googleapis.com/syzbot-assets/8d47fe92aaa8/bzImage-cf7c3c02.xz
>
>IMPORTANT: if you fix the issue, please add the following tag to the commit:
>Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com
>
> free_pages_and_swap_cache+0x2b9/0x490 mm/swap_state.c:401
> __tlb_batch_free_encoded_pages mm/mmu_gather.c:138 [inline]
> tlb_batch_pages_flush mm/mmu_gather.c:151 [inline]
> tlb_flush_mmu_free mm/mmu_gather.c:417 [inline]
> tlb_flush_mmu+0x6d3/0xa30 mm/mmu_gather.c:424
> tlb_finish_mmu+0xf9/0x230 mm/mmu_gather.c:549
> exit_mmap+0x498/0x9e0 mm/mmap.c:1313
> __mmput+0x118/0x430 kernel/fork.c:1177
> exit_mm+0x18e/0x250 kernel/exit.c:581
> do_exit+0x6a2/0x22c0 kernel/exit.c:962
> do_group_exit+0x21b/0x2d0 kernel/exit.c:1116
> __do_sys_exit_group kernel/exit.c:1127 [inline]
> __se_sys_exit_group kernel/exit.c:1125 [inline]
> __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1125
> x64_sys_call+0x221a/0x2240 arch/x86/include/generated/asm/syscalls_64.h:232
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>------------[ cut here ]------------
>1
>WARNING: mm/huge_memory.c:4371 at deferred_split_folio+0x974/0xaa0 mm/huge_memory.c:4371, CPU#1: syz.3.1110/10500
>Modules linked in:
>CPU: 1 UID: 0 PID: 10500 Comm: syz.3.1110 Not tainted syzkaller #0 PREEMPT(full) 
>Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
>RIP: 0010:deferred_split_folio+0x974/0xaa0 mm/huge_memory.c:4371
>Code: 31 48 8d 65 d8 5b 41 5c 41 5d 41 5e 41 5f 5d e9 c2 67 8d 09 cc e8 8c 73 93 ff 48 89 df 48 c7 c6 20 5b fc 8b e8 dd 2b f5 fe 90 <0f> 0b 90 e9 d4 fe ff ff e8 9f 7a 8a 09 e8 6a 73 93 ff 48 89 df 48
>RSP: 0018:ffffc900047ef540 EFLAGS: 00010046
>RAX: 1c05fb65cfaab100 RBX: ffffea0001840000 RCX: 0000000080000001
>RDX: 0000000000000002 RSI: ffffffff8e4da1c7 RDI: ffff88807d6f9e80
>RBP: ffffc900047ef610 R08: ffff8880b87247d3 R09: 1ffff110170e48fa
>R10: dffffc0000000000 R11: ffffed10170e48fb R12: ffffea0001840040
>R13: 0000000000000000 R14: 0000000000010000 R15: 1ffff920008fdeb0
>FS:  00007f32e32a76c0(0000) GS:ffff8881250e8000(0000) knlGS:0000000000000000
>CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>CR2: 00007f5825757930 CR3: 0000000034ad8000 CR4: 00000000003526f0
>Call Trace:
> <TASK>
> migrate_folio_move mm/migrate.c:1411 [inline]

Looks like a race introduced by commit[1] ("mm: migrate: requeue
destination folio on deferred split queue").

Between folio migration (mbind) and rmap removal (exit_mmap), I guess :)

migrate_folio_move() snapshots src_partially_mapped from src before
migration:

	if (folio_order(src) > 1 &&
	    !data_race(list_empty(&src->_deferred_list))) {
		src_deferred_split = true;
		src_partially_mapped = folio_test_partially_mapped(src);
	}

Then move_to_new_folio() eventually unqueues src in
__folio_migrate_mapping():

	folio_unqueue_deferred_split(src);

After that, migration restores mappings to dst:

	if (old_page_state & PAGE_WAS_MAPPED)
		remove_migration_ptes(src, dst, 0);

At that point, dst is already visible again. A concurrent unmap path
from another sharer can then remove some of those mappings and reach
deferred_split_folio(dst, true), which sets PG_partially_mapped on
dst.

Migration then resumes and does:

	if (src_deferred_split)
		deferred_split_folio(dst, src_partially_mapped);

If the earlier snapshot from src was false, this becomes
deferred_split_folio(dst, false), but dst may already have been marked
partially mapped by the concurrent rmap-removal path, so the WARN in
deferred_split_folio() fires:

	if (partially_mapped) {
		...
	} else {
		/* partially mapped folios cannot become non-partially mapped */
		VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
	}

[1] https://lore.kernel.org/all/20260312104723.1351321-1-usama.arif@linux.dev/

> migrate_folios_move mm/migrate.c:1740 [inline]
> migrate_pages_batch+0x319f/0x4c40 mm/migrate.c:1996
> migrate_pages_sync mm/migrate.c:2026 [inline]
> migrate_pages+0x1c74/0x2a10 mm/migrate.c:2135
> do_mbind mm/mempolicy.c:1614 [inline]
> kernel_mbind mm/mempolicy.c:1757 [inline]
> __do_sys_mbind mm/mempolicy.c:1831 [inline]
> __se_sys_mbind+0xe89/0x10f0 mm/mempolicy.c:1827
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>RIP: 0033:0x7f32e239c819
>Code: ff 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 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
>RSP: 002b:00007f32e32a7028 EFLAGS: 00000246 ORIG_RAX: 00000000000000ed
>RAX: ffffffffffffffda RBX: 00007f32e2616090 RCX: 00007f32e239c819
>RDX: 0000000000000000 RSI: 0000000000800000 RDI: 0000200000001000
>RBP: 00007f32e2432c91 R08: 0000000000000000 R09: 0000000000000002
>R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
>R13: 00007f32e2616128 R14: 00007f32e2616090 R15: 00007ffe30c61cc8
> </TASK>
>
>
>---
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