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

[David Hildenbrand <david@redhat.com>]

On 20.05.25 07:45, Shivank Garg wrote:
> On 5/19/2025 6:56 PM, David Hildenbrand wrote:
>> On 17.05.25 10:21, syzbot wrote:
>>> Hello,
>>>
>>> syzbot found the following issue on:
>>>
>>> HEAD commit:    627277ba7c23 Merge tag 'arm64_cbpf_mitigation_2025_05_08' ..
>>> git tree:       upstream
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=1150f670580000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=5929ac65be9baf3c
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=2b99589e33edbe9475ca
>>> compiler:       Debian clang version 20.1.2 (++20250402124445+58df0ef89dd6-1~exp1~20250402004600.97), Debian LLD 20.1.2
>>>
>>> Unfortunately, I don't have any reproducer for this issue yet.
>>>
>>> Downloadable assets:
>>> disk image: https://storage.googleapis.com/syzbot-assets/0a42ae72fe0e/disk-627277ba.raw.xz
>>> vmlinux: https://storage.googleapis.com/syzbot-assets/0be88297bb66/vmlinux-627277ba.xz
>>> kernel image: https://storage.googleapis.com/syzbot-assets/31808a4b1210/bzImage-627277ba.xz
>>>
>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>> Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com
>>>
>>> ------------[ cut here ]------------
>>> WARNING: CPU: 1 PID: 38 at ./include/linux/mm.h:1335 folio_large_mapcount+0xd0/0x110 include/linux/mm.h:1335
>>
>> This should be
>>
>> VM_WARN_ON_FOLIO(!folio_test_large(folio), folio);
>>
>>> Modules linked in:
>>> CPU: 1 UID: 0 PID: 38 Comm: khugepaged Not tainted 6.15.0-rc6-syzkaller-00025-g627277ba7c23 #0 PREEMPT(full)
>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
>>> RIP: 0010:folio_large_mapcount+0xd0/0x110 include/linux/mm.h:1335
>>> Code: 04 38 84 c0 75 29 8b 03 ff c0 5b 41 5e 41 5f e9 96 d2 2b 09 cc e8 d0 cb 99 ff 48 89 df 48 c7 c6 20 de 77 8b e8 a1 dc de ff 90 <0f> 0b 90 eb b6 89 d9 80 e1 07 80 c1 03 38 c1 7c cb 48 89 df e8 87
>>> RSP: 0018:ffffc90000af77e0 EFLAGS: 00010246
>>> RAX: e1fcb38c0ff8ce00 RBX: ffffea00014c8000 RCX: e1fcb38c0ff8ce00
>>> RDX: 0000000000000001 RSI: ffffffff8d9226df RDI: ffff88801e2fbc00
>>> RBP: ffffc90000af7b50 R08: ffff8880b8923e93 R09: 1ffff110171247d2
>>> R10: dffffc0000000000 R11: ffffed10171247d3 R12: 1ffffd4000299000
>>> R13: dffffc0000000000 R14: 0000000000000000 R15: dffffc0000000000
>>> FS:  0000000000000000(0000) GS:ffff8881261fb000(0000) knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 00007ffe58f12dc0 CR3: 0000000030e04000 CR4: 00000000003526f0
>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>> Call Trace:
>>>    <TASK>
>>>    folio_mapcount include/linux/mm.h:1369 [inline]
>>
>> And here we come through
>>
>> if (likely(!folio_test_large(folio))) {
>>      ...
>> }
>> return folio_large_mapcount(folio);
>>
>>
>> So the folio is split concurrently. And I think there is nothing stopping it from getting freed.
>>
>> We do a xas_for_each() under RCU. So yes, this is racy.
>>
>> In  collapse_file(), we re-validate everything.
>>
>> We could
>>
>> (A) Take proper pagecache locks
>>
>> (B) Try grabbing a temporary folio reference
>>
>> (C) Try snapshotting the folio
>>
>> Probably, in this code, (B) might be cleanest for now? Handling it just like other code in mm/filemap.c.
>>
> 
> Hi,

Hi,

> 
> I've implemented your suggestion (B) using folio_try_get().
> Could you please review if my patch looks correct?

You should probably drop both comments, the code merely mimics what 
filemap.c does.

Apart from that, nothing jumped at me.

-- 
Cheers,

David / dhildenb