Message-ID: <CAJnrk1Ziam4ZqqyzOpbUD8j=RwJOK22Uz3VMqWZsUNiJ5bkBrg@mail.gmail.com>
Date: Wed, 27 Aug 2025 09:42:01 -0700
From: Joanne Koong <joannelkoong@...il.com>
To: Aleksandr Nogikh <nogikh@...gle.com>
Cc: syzbot <syzbot+0630e71306742d4b2aea@...kaller.appspotmail.com>, 
	David Hildenbrand <david@...hat.com>, mszeredi@...hat.com, akpm@...ux-foundation.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, linux-next@...r.kernel.org, sfr@...b.auug.org.au, 
	syzkaller-bugs@...glegroups.com, willy@...radead.org, 
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [syzbot] [fs?] [mm?] linux-next test error: WARNING in __folio_start_writeback

On Wed, Aug 27, 2025 at 6:45 AM Aleksandr Nogikh <nogikh@...gle.com> wrote:
>
> I've bisected the problem to the following commit:
>
> commit 167f21a81a9c4dbd6970a4ee3853aecad405fa7f (HEAD)
> Author: Joanne Koong <joannelkoong@...il.com>
> Date:   Mon Jul 7 16:46:06 2025 -0700
>
>     mm: remove BDI_CAP_WRITEBACK_ACCT
>
>     There are no users of BDI_CAP_WRITEBACK_ACCT now that fuse doesn't do
>     its own writeback accounting. This commit removes
>     BDI_CAP_WRITEBACK_ACCT.
>
> Joanne Koong, could you please take a look at the syzbot report below?

Hi Aleksandr,

Thanks for bisecting this. This is a duplicate of what Marek reported
in [1]. His patch in [2] fixes the warning getting triggered.

Marek, could you submit your patch formally to the mm tree so it could
be picked up?


Thanks,
Joanne


[1] https://lore.kernel.org/linux-fsdevel/a91010a8-e715-4f3d-9e22-e4c34efc0408@samsung.com/T/#u
[2] https://lore.kernel.org/linux-fsdevel/a91010a8-e715-4f3d-9e22-e4c34efc0408@samsung.com/T/#m3aa6506ee7de302242e64861f8e2199f24e4ad46

>
> On Fri, Aug 22, 2025 at 12:09 PM syzbot
> <syzbot+0630e71306742d4b2aea@...kaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    0f4c93f7eb86 Add linux-next specific files for 20250822
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=172c07bc580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=21eed27c0deadb92
> > dashboard link: https://syzkaller.appspot.com/bug?extid=0630e71306742d4b2aea
> > compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/669ede8f5d66/disk-0f4c93f7.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/50feda89fe89/vmlinux-0f4c93f7.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/317a0d3516fb/bzImage-0f4c93f7.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+0630e71306742d4b2aea@...kaller.appspotmail.com
> >
> > ------------[ cut here ]------------
> > WARNING: ./include/linux/backing-dev.h:243 at inode_to_wb include/linux/backing-dev.h:239 [inline], CPU#1: kworker/u8:6/2949
> > WARNING: ./include/linux/backing-dev.h:243 at __folio_start_writeback+0x9d5/0xb70 mm/page-writeback.c:3027, CPU#1: kworker/u8:6/2949
> > Modules linked in:
> > CPU: 1 UID: 0 PID: 2949 Comm: kworker/u8:6 Not tainted syzkaller #0 PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
> > Workqueue: writeback wb_workfn (flush-8:0)
> > RIP: 0010:inode_to_wb include/linux/backing-dev.h:239 [inline]
> > RIP: 0010:__folio_start_writeback+0x9d5/0xb70 mm/page-writeback.c:3027
> > Code: 28 4c 89 f8 48 c1 e8 03 42 80 3c 28 00 74 08 4c 89 ff e8 ce a2 29 00 49 8b 07 25 ff 3f 00 00 e9 1b fa ff ff e8 7c 04 c6 ff 90 <0f> 0b 90 e9 d6 fb ff ff e8 6e 04 c6 ff 48 c7 c7 a0 f8 5f 8e 4c 89
> > RSP: 0018:ffffc9000bb06ea0 EFLAGS: 00010293
> > RAX: ffffffff81fad344 RBX: ffffea00050de8c0 RCX: ffff88802ee29e00
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> > RBP: ffffc9000bb07010 R08: ffffc9000bb06f97 R09: 0000000000000000
> > R10: ffffc9000bb06f80 R11: fffff52001760df3 R12: ffffea00050de8c8
> > R13: 0000000000000000 R14: ffff888023060880 R15: ffff888023060660
> > FS:  0000000000000000(0000) GS:ffff8881258c3000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007f7354907000 CR3: 000000000e338000 CR4: 00000000003526f0
> > Call Trace:
> >  <TASK>
> >  __block_write_full_folio+0x75f/0xe10 fs/buffer.c:1928
> >  blkdev_writepages+0xd1/0x170 block/fops.c:484
> >  do_writepages+0x32e/0x550 mm/page-writeback.c:2604
> >  __writeback_single_inode+0x145/0xff0 fs/fs-writeback.c:1680
> >  writeback_sb_inodes+0x6c7/0x1010 fs/fs-writeback.c:1976
> >  __writeback_inodes_wb+0x111/0x240 fs/fs-writeback.c:2047
> >  wb_writeback+0x44f/0xaf0 fs/fs-writeback.c:2158
> >  wb_check_old_data_flush fs/fs-writeback.c:2262 [inline]
> >  wb_do_writeback fs/fs-writeback.c:2315 [inline]
> >  wb_workfn+0xaef/0xef0 fs/fs-writeback.c:2343
> >  process_one_work kernel/workqueue.c:3236 [inline]
> >  process_scheduled_works+0xade/0x17b0 kernel/workqueue.c:3319
> >  worker_thread+0x8a0/0xda0 kernel/workqueue.c:3400
> >  kthread+0x711/0x8a0 kernel/kthread.c:463
> >  ret_from_fork+0x47c/0x820 arch/x86/kernel/process.c:148
> >  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> >  </TASK>
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@...glegroups.com.
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> >
> > If the report is already addressed, let syzbot know by replying with:
> > #syz fix: exact-commit-title
> >
> > If you want to overwrite report's subsystems, reply with:
> > #syz set subsystems: new-subsystem
> > (See the list of subsystem names on the web dashboard)
> >
> > If the report is a duplicate of another one, reply with:
> > #syz dup: exact-subject-of-another-report
> >
> > If you want to undo deduplication, reply with:
> > #syz undup
> >

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