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From:   Yang Shi <shy828301@gmail.com>
Date:   Tue, 24 Aug 2021 12:09:45 -0700
Message-ID: <CAHbLzkrKRUymXEz6EFQ4RafQByEgGxNe278o=xaqtMgbxjM7sg@mail.gmail.com>
Subject: Re: [syzbot] WARNING in split_huge_page_to_list
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     syzbot <syzbot+0c65e03871b364e714a3@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>, syzkaller-bugs@googlegroups.com,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>
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On Tue, Aug 24, 2021 at 4:41 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> That's VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
> CC folks involved in 504e070dc08f ("mm: thp: replace DEBUG_VM BUG with VM_WARN
> when unmap fails for split")

There might be race between THP split and page unmap, and THP split
code can handle it, so it is not worth a VM_BUG.

Hugh fixed a couple of potential races, but there might be others.

>
> On 8/20/21 18:48, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    f87d64319e6f Merge tag 'net-5.14-rc7' of git://git.kernel...
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1205bd55300000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=3205625db2f96ac9
> > dashboard link: https://syzkaller.appspot.com/bug?extid=0c65e03871b364e714a3
> > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+0c65e03871b364e714a3@syzkaller.appspotmail.com
> >
> >  __do_munmap+0x627/0x11c0 mm/mmap.c:2911
> >  do_munmap mm/mmap.c:2922 [inline]
> >  munmap_vma_range mm/mmap.c:604 [inline]
> >  mmap_region+0x85a/0x1760 mm/mmap.c:1753
> >  do_mmap+0x86e/0x1180 mm/mmap.c:1584
> >  vm_mmap_pgoff+0x1b7/0x290 mm/util.c:519
> >  ksys_mmap_pgoff+0x4a8/0x620 mm/mmap.c:1635
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > ------------[ cut here ]------------
> > WARNING: CPU: 0 PID: 2144 at mm/huge_memory.c:2327 unmap_page mm/huge_memory.c:2327 [inline]
> > WARNING: CPU: 0 PID: 2144 at mm/huge_memory.c:2327 split_huge_page_to_list+0x25eb/0x3e60 mm/huge_memory.c:2700
> > Modules linked in:
> > CPU: 0 PID: 2144 Comm: kswapd1 Not tainted 5.14.0-rc6-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > RIP: 0010:unmap_page mm/huge_memory.c:2327 [inline]
> > RIP: 0010:split_huge_page_to_list+0x25eb/0x3e60 mm/huge_memory.c:2700
> > Code: 89 de e8 28 e1 b6 ff 84 db 0f 85 54 e0 ff ff e8 6b d9 b6 ff 48 c7 c6 e0 d8 97 89 48 89 ef e8 9c 72 e9 ff c6 05 05 dd 99 0b 01 <0f> 0b e9 32 e0 ff ff bb 02 00 00 00 e9 64 ef ff ff e8 3f d9 b6 ff
> > RSP: 0018:ffffc900084f76d8 EFLAGS: 00010293
> > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> > RDX: ffff88801c44c000 RSI: ffffffff81be0514 RDI: 0000000000000003
> > RBP: ffffea00021c0000 R08: 0000000000000029 R09: 00000000ffffffff
> > R10: ffffffff88ea7679 R11: 00000000ffffffff R12: ffff88814018d2d8
> > R13: ffffea00021c0008 R14: ffffea00021c0000 R15: 0000000000000000
> > FS:  0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007f14895d5000 CR3: 0000000017350000 CR4: 00000000001506f0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> >  split_huge_page include/linux/huge_mm.h:192 [inline]
> >  deferred_split_scan+0x5fa/0xbd0 mm/huge_memory.c:2860
> >  do_shrink_slab+0x42d/0xbd0 mm/vmscan.c:709
> >  shrink_slab+0x17c/0x6e0 mm/vmscan.c:869
> >  shrink_node_memcgs mm/vmscan.c:2868 [inline]
> >  shrink_node+0x8d1/0x1df0 mm/vmscan.c:2983
> >  kswapd_shrink_node mm/vmscan.c:3726 [inline]
> >  balance_pgdat+0x7ce/0x13b0 mm/vmscan.c:3917
> >  kswapd+0x5b6/0xdb0 mm/vmscan.c:4176
> >  kthread+0x3e5/0x4d0 kernel/kthread.c:319
> >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> >
> >
> > ---
> > 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@googlegroups.com.
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> >
>