Re: [syzbot] [mm?] KCSAN: data-race in try_to_compact_pages / try_to_compact_pages

[Vlastimil Babka <vbabka@suse.cz>]

On 9/26/25 08:45, syzbot wrote:
> 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=12448f12580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=6e0c213d0735f5dd
> dashboard link: https://syzkaller.appspot.com/bug?extid=20dd68d4ba19e8242c26
> 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+20dd68d4ba19e8242c26@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KCSAN: data-race in try_to_compact_pages / try_to_compact_pages
> 
> read-write to 0xffff88823fffacf0 of 4 bytes by task 6757 on cpu 1:
>  compaction_deferred mm/compaction.c:149 [inline]

Yeah data races for zone->compact_considered and zone->compact_order_failed
can happen but are not a problem. Somebody motivated enough might annotate them.

>  try_to_compact_pages+0x1be/0x940 mm/compaction.c:2838
>  __alloc_pages_direct_compact+0x65/0x1d0 mm/page_alloc.c:4066
>  __alloc_pages_slowpath+0x360/0x5f0 mm/page_alloc.c:4787
>  __alloc_frozen_pages_noprof+0x270/0x360 mm/page_alloc.c:5161
>  alloc_pages_mpol+0xb3/0x250 mm/mempolicy.c:2416
>  alloc_frozen_pages_noprof+0x90/0x110 mm/mempolicy.c:2487
>  ___kmalloc_large_node+0x52/0x100 mm/slub.c:4317
>  __kmalloc_large_node_noprof+0x16/0xa0 mm/slub.c:4348
>  __do_kmalloc_node mm/slub.c:4364 [inline]
>  __kmalloc_noprof+0x2ab/0x3e0 mm/slub.c:4388
>  kmalloc_noprof include/linux/slab.h:909 [inline]
>  kmalloc_array_noprof include/linux/slab.h:948 [inline]
>  io_vec_realloc io_uring/rsrc.c:1327 [inline]
>  io_import_reg_vec+0x645/0xe80 io_uring/rsrc.c:1520
>  io_sendmsg_zc+0x91/0x550 io_uring/net.c:1545
>  __io_issue_sqe+0xfb/0x2e0 io_uring/io_uring.c:1771
>  io_issue_sqe+0x53/0x970 io_uring/io_uring.c:1794
>  io_queue_sqe io_uring/io_uring.c:2023 [inline]
>  io_submit_sqe io_uring/io_uring.c:2283 [inline]
>  io_submit_sqes+0x675/0x1060 io_uring/io_uring.c:2396
>  __do_sys_io_uring_enter io_uring/io_uring.c:3463 [inline]
>  __se_sys_io_uring_enter+0x1c1/0x1b70 io_uring/io_uring.c:3397
>  __x64_sys_io_uring_enter+0x78/0x90 io_uring/io_uring.c:3397
>  x64_sys_call+0x2de1/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:427
>  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 0xffff88823fffacf0 of 4 bytes by task 6696 on cpu 0:
>  compaction_deferred mm/compaction.c:150 [inline]
>  try_to_compact_pages+0x207/0x940 mm/compaction.c:2838
>  __alloc_pages_direct_compact+0x65/0x1d0 mm/page_alloc.c:4066
>  __alloc_pages_slowpath+0x360/0x5f0 mm/page_alloc.c:4787
>  __alloc_frozen_pages_noprof+0x270/0x360 mm/page_alloc.c:5161
>  alloc_pages_mpol+0xb3/0x250 mm/mempolicy.c:2416
>  alloc_frozen_pages_noprof+0x90/0x110 mm/mempolicy.c:2487
>  ___kmalloc_large_node+0x52/0x100 mm/slub.c:4317
>  __kmalloc_large_node_noprof+0x16/0xa0 mm/slub.c:4348
>  __do_kmalloc_node mm/slub.c:4364 [inline]
>  __kmalloc_noprof+0x2ab/0x3e0 mm/slub.c:4388
>  kmalloc_noprof include/linux/slab.h:909 [inline]
>  kmalloc_array_noprof include/linux/slab.h:948 [inline]
>  io_vec_realloc io_uring/rsrc.c:1327 [inline]
>  io_import_reg_vec+0x645/0xe80 io_uring/rsrc.c:1520
>  io_sendmsg_zc+0x91/0x550 io_uring/net.c:1545
>  __io_issue_sqe+0xfb/0x2e0 io_uring/io_uring.c:1771
>  io_issue_sqe+0x53/0x970 io_uring/io_uring.c:1794
>  io_queue_sqe io_uring/io_uring.c:2023 [inline]
>  io_submit_sqe io_uring/io_uring.c:2283 [inline]
>  io_submit_sqes+0x675/0x1060 io_uring/io_uring.c:2396
>  __do_sys_io_uring_enter io_uring/io_uring.c:3463 [inline]
>  __se_sys_io_uring_enter+0x1c1/0x1b70 io_uring/io_uring.c:3397
>  __x64_sys_io_uring_enter+0x78/0x90 io_uring/io_uring.c:3397
>  x64_sys_call+0x2de1/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:427
>  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: 0x00000005 -> 0x00000004
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 UID: 0 PID: 6696 Comm: syz.2.1234 Not tainted syzkaller #0 PREEMPT(voluntary) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
> ==================================================================
> 
> 
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