[syzbot] [mm?] KCSAN: data-race in try_to_migrate_one / zap_page_range_single_batched

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: cec1e6e5d1ab Merge tag 'sched_ext-for-6.17-rc7-fixes' of g..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=145d4f12580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6e0c213d0735f5dd
dashboard link: https://syzkaller.appspot.com/bug?extid=60192c8877d0bc92a92b
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/10b7c8fdfdec/disk-cec1e6e5.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/cbecc36962db/vmlinux-cec1e6e5.xz
kernel image: https://storage.googleapis.com/syzbot-assets/214f107d0a3e/bzImage-cec1e6e5.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+60192c...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in try_to_migrate_one / zap_page_range_single_batched

write to 0xffff88810adfd798 of 8 bytes by task 13594 on cpu 1:
update_hiwater_rss include/linux/mm.h:2657 [inline]
try_to_migrate_one+0x918/0x16e0 mm/rmap.c:2455
__rmap_walk_file+0x1ec/0x2b0 mm/rmap.c:2905
try_to_migrate+0x1db/0x210 mm/rmap.c:-1
migrate_folio_unmap mm/migrate.c:1324 [inline]
migrate_pages_batch+0x6e1/0x1ae0 mm/migrate.c:1873
migrate_pages_sync mm/migrate.c:1996 [inline]
migrate_pages+0xf5f/0x1770 mm/migrate.c:2105
do_mbind mm/mempolicy.c:1539 [inline]
kernel_mbind mm/mempolicy.c:1682 [inline]
__do_sys_mbind mm/mempolicy.c:1756 [inline]
__se_sys_mbind+0x975/0xac0 mm/mempolicy.c:1752
__x64_sys_mbind+0x78/0x90 mm/mempolicy.c:1752
x64_sys_call+0x2932/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:238
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

write to 0xffff88810adfd798 of 8 bytes by task 13595 on cpu 0:
update_hiwater_rss include/linux/mm.h:2657 [inline]
zap_page_range_single_batched+0x182/0x450 mm/memory.c:2007
zap_page_range_single mm/memory.c:2041 [inline]
unmap_mapping_range_vma mm/memory.c:4020 [inline]
unmap_mapping_range_tree+0xfd/0x160 mm/memory.c:4037
unmap_mapping_pages mm/memory.c:4103 [inline]
unmap_mapping_range+0xe4/0xf0 mm/memory.c:4140
shmem_fallocate+0x262/0x840 mm/shmem.c:3746
vfs_fallocate+0x3b6/0x400 fs/open.c:342
madvise_remove mm/madvise.c:1049 [inline]
madvise_vma_behavior+0x192d/0x1cf0 mm/madvise.c:1346
madvise_walk_vmas mm/madvise.c:1669 [inline]
madvise_do_behavior+0x5b7/0x970 mm/madvise.c:1885
do_madvise+0x10e/0x190 mm/madvise.c:1978
__do_sys_madvise mm/madvise.c:1987 [inline]
__se_sys_madvise mm/madvise.c:1985 [inline]
__x64_sys_madvise+0x64/0x80 mm/madvise.c:1985
x64_sys_call+0x1f1a/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:29
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x0000000000001645 -> 0x0000000000002165

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 13595 Comm: syz.1.3492 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
==================================================================


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[Marco Elver]

On Fri, 26 Sept 2025 at 08:44, syzbot

One of these writes is getting lost. Which means highwater_rss is
lossy/approximate - does it matter?

[Lance Yang]

Yes. IMO, hiwater_rss being lossy is an acceptable trade-off for a
non-critical accounting value. The patch[1] simply silences the KCSAN warning.

[1] https://lore.kernel.org/linux-mm/20250926092426.4...@linux.dev/