Re: [syzbot] [mm?] BUG: Bad page state in iomap_write_begin

[David Hildenbrand <david@redhat.com>]

On 20.11.24 09:37, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    e8bdb3c8be08 Merge tag 'riscv-for-linus-6.12-rc8' of git:/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1644d2c0580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d2aeec8c0b2e420c
> dashboard link: https://syzkaller.appspot.com/bug?extid=c317c107c68f8bc257d9
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1144d2c0580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=176252e8580000
> 
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-e8bdb3c8.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/fcae16ff0e0f/vmlinux-e8bdb3c8.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/758973beace8/bzImage-e8bdb3c8.xz
> mounted in repro #1: https://storage.googleapis.com/syzbot-assets/a097a300c8a9/mount_0.gz
> mounted in repro #2: https://storage.googleapis.com/syzbot-assets/36b9fff6ab5e/mount_6.gz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+c317c107c68f8bc257d9@syzkaller.appspotmail.com
> 
> BUG: Bad page state in process syz-executor315  pfn:49e01
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x49e01
> head: order:0 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:-1

So, we have a pincount underflow on a large folio (order-9). We 
allocated the folio in iomap_write_begin()->__iomap_get_folio(). Nothing 
special.

[...]

>   </TASK>
> BUG: Bad page state in process syz-executor315  pfn:49e00
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x49e00
> head: order:0 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> flags: 0x4fff0000000004d(locked|referenced|uptodate|head|node=1|zone=1|lastcpupid=0x7ff)
> raw: 04fff0000000004d dead000000000100 dead000000000122 0000000000000000
> raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
> head: 04fff0000000004d dead000000000100 dead000000000122 0000000000000000
> head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
> head: 04fff00000000000 0000000000000000 ffffffffffffffff 0000000000000000
> head: 0000000000000200 0000000000000000 00000000ffffffff 0000000000000000
> page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set

And we seem to have an unexpected flag set on the head page of the same 
large folio. Likely PG_locked.

...

> XFS (loop0): Unmounting Filesystem 9f1cad42-11bd-4e12-8f0b-f07876b81d9a
> BUG: Bad page state in process syz-executor315  pfn:49c01
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x49c01
> head: order:0 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:-1
> flags: 0x4fff0000000004d(locked|referenced|uptodate|head|node=1|zone=1|lastcpupid=0x7ff)
> raw: 04fff00000000000 ffffea0001270001 ffffffffffffffff ffffffff00000000
> raw: 0000000000000200 0000000000000000 00000000ffffffff 0000000000000000
> head: 04fff0000000004d dead000000000100 dead000000000122 0000000000000000
> head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
> head: 04fff00000000000 ffffea0001270001 ffffffffffffffff ffffffff00000000
> head: 0000000000000200 0000000000000000 00000000ffffffff 0000000000000000
> page dumped because: nonzero pincount

[...]

>   </TASK>
> BUG: Bad page state in process syz-executor315  pfn:49c00
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x49c00
> head: order:0 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> flags: 0x4fff0000000004d(locked|referenced|uptodate|head|node=1|zone=1|lastcpupid=0x7ff)
> raw: 04fff0000000004d dead000000000100 dead000000000122 0000000000000000
> raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
> head: 04fff0000000004d dead000000000100 dead000000000122 0000000000000000
> head: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
> head: 04fff00000000000 0000000000000000 ffffffffffffffff 0000000000000000
> head: 0000000000000200 0000000000000000 00000000ffffffff 0000000000000000
> page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
> page_owner tracks the page as allocated

Same thing, different folio.


So we seem to be repeatedly freeing a large folio that
(a) Is locked
(b) Has a pincount underflow

Likely because we have an unbalanced unpin, that happens before 
evicting/truncating the folio. If we then evict/truncate the (locked) 
folio, we free the folio before unlocking it and dropping the actual 
last reference.

Not sure why we see a comparable thing with small folios (pincount part 
of refcount, but on unpin our refcount should seriously underflow 
immediately)

CCing David

-- 
Cheers,

David / dhildenb