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

Zi Yan
2025-09-24T15:03:17Z

On 24 Sep 2025, at 7:32, David Hildenbrand wrote:

> On 23.09.25 18:22, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit: b5db4add5e77 Merge branch 'for-next/core' into for-kernelci
>> git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
>> console output: https://syzkaller.appspot.com/x/log.txt?x=10edb8e2580000
>> kernel config: https://syzkaller.appspot.com/x/.config?x=d2ae34a0711ff2f1
>> dashboard link: https://syzkaller.appspot.com/bug?extid=e6367ea2fdab6ed46056
>> compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
>> userspace arch: arm64
>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14160f12580000
>> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1361627c580000
>>
>> Downloadable assets:
>> disk image: https://storage.googleapis.com/syzbot-assets/6eee2232d5c1/disk-b5db4add.raw.xz
>> vmlinux: https://storage.googleapis.com/syzbot-assets/a8b00f2f1234/vmlinux-b5db4add.xz
>> kernel image: https://storage.googleapis.com/syzbot-assets/fc0d466f156c/Image-b5db4add.gz.xz
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+e6367ea2fdab6ed46056@syzkaller.appspotmail.com
>>
>> Injecting memory failure for pfn 0x104000 at process virtual address 0x20000000
>> ------------[ cut here ]------------
>> WARNING: CPU: 1 PID: 6700 at mm/memory-failure.c:2391 memory_failure+0x18ec/0x1db4 mm/memory-failure.c:2391
>> Modules linked in:
>> CPU: 1 UID: 0 PID: 6700 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/30/2025
>> pstate: 83400005 (Nzcv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
>> pc : memory_failure+0x18ec/0x1db4 mm/memory-failure.c:2391
>> lr : memory_failure+0x18ec/0x1db4 mm/memory-failure.c:2391
>> sp : ffff8000a41478c0
>> x29: ffff8000a41479a0 x28: 05ffc00000200868 x27: ffff700014828f20
>> x26: 1fffffbff8620001 x25: 05ffc0000020086d x24: 1fffffbff8620000
>> x23: fffffdffc3100008 x22: fffffdffc3100000 x21: fffffdffc3100000
>> x20: 0000000000000023 x19: dfff800000000000 x18: 1fffe00033793888
>> x17: ffff80008f7ee000 x16: ffff80008052aa64 x15: 0000000000000001
>> x14: 1fffffbff8620000 x13: 0000000000000000 x12: 0000000000000000
>> x11: ffff7fbff8620001 x10: 0000000000ff0100 x9 : 0000000000000000
>> x8 : ffff0000d7eedb80 x7 : ffff800080428910 x6 : 0000000000000000
>> x5 : 0000000000000001 x4 : 0000000000000001 x3 : ffff800080cf5438
>> x2 : 0000000000000001 x1 : 0000000000000040 x0 : 0000000000000000
>> Call trace:
>> memory_failure+0x18ec/0x1db4 mm/memory-failure.c:2391 (P)
>> madvise_inject_error mm/madvise.c:1475 [inline]
>> madvise_do_behavior+0x2c8/0x7c4 mm/madvise.c:1875
>> do_madvise+0x190/0x248 mm/madvise.c:1978
>> __do_sys_madvise mm/madvise.c:1987 [inline]
>> __se_sys_madvise mm/madvise.c:1985 [inline]
>> __arm64_sys_madvise+0xa4/0xc0 mm/madvise.c:1985
>> __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
>> invoke_syscall+0x98/0x254 arch/arm64/kernel/syscall.c:49
>> el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:132
>> do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
>> el0_svc+0x5c/0x254 arch/arm64/kernel/entry-common.c:744
>> el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:763
>> el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:596
>
> We're running into the
>
> WARN_ON(folio_test_large(folio));
>
> in memory_failure().
>
> Which is weird because we have the
>
> if (folio_test_large(folio)) {
> /*
> * The flag must be set after the refcount is bumped
> * otherwise it may race with THP split.
> * And the flag can't be set in get_hwpoison_page() since
> * it is called by soft offline too and it is just called
> * for !MF_COUNT_INCREASED. So here seems to be the best
> * place.
> *
> * Don't need care about the above error handling paths for
> * get_hwpoison_page() since they handle either free page
> * or unhandlable page. The refcount is bumped iff the
> * page is a valid handlable page.
> */
> folio_set_has_hwpoisoned(folio);
> if (try_to_split_thp_page(p, false) < 0) {
> res = -EHWPOISON;
> kill_procs_now(p, pfn, flags, folio);
> put_page(p);
> action_result(pfn, MF_MSG_UNSPLIT_THP, MF_FAILED);
> goto unlock_mutex;
> }
> VM_BUG_ON_PAGE(!page_count(p), p);
> folio = page_folio(p);
> }
>
> before it.
>
> But likely that's what I raised to Zi Yan recently: if try_to_split_thp_page()->split_huge_page()
> silently decided to split to something that is not a small folio (the min_order_for_split() bit),
> this changed the semantics of the function.
>
> Likely split_huge_page() should have failed if the min_order makes us not split to order-0,
> or there would have to be some "parameter" that tells split_huge_page() what expectation (order) the
> callers has.
>
> We can check folio_test_large() after the split, but really, we should just not be splitting at
> all if it doesn't serve our purpose.

But LBS might want to split from a high order to fs min_order.

What I can think of is:
0. split code always does a split to allowed minimal order,
namely max(fs_min_order, order_from_caller);
1. if split order cannot reach to order_from_caller, it just return fails,
so most of the caller will know about it;
2. for LBS code, when it sees a split failure, it should check the resulting
folio order against fs min_order. If the orders match, it regards it as
a success.

At least, most of the code does not need to be LBS aware. WDYT?

Best Regards,
Yan, Zi