linux-kernel - Is there a reason why REQ_OP

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Message-ID: <2117829.1673884910@warthog.procyon.org.uk>
Date:   Mon, 16 Jan 2023 16:01:50 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@...radead.org>
cc:     dhowells@...hat.com
Subject: Is there a reason why REQ_OP_READ has to be 0?

Hi Jens, Christoph,

Do you know if there's a reason why REQ_OP_READ has to be 0?  I'm seeing a
circumstance where a direct I/O write on a blockdev is BUG'ing in my modified
iov_iter code because the iterator says it's a source iterator (correct), but
the bio->bi_opf == REQ_OP_READ (which should be wrong).

I thought I'd move REQ_OP_READ to, say, 4 so that I could try and see if it's
just undefined but the kernel BUGs and then panics during boot.

David

------------[ cut here ]------------
kernel BUG at mm/filemap.c:1615!
------------[ cut here ]------------
invalid opcode: 0000 [#1] PREEMPT SMP PTI
kernel BUG at mm/filemap.c:1615!
CPU: 1 PID: 2196 Comm: systemd-udevd Not tainted 6.2.0-rc2-build3+ #12783
Hardware name: ASUS All Series/H97-PLUS, BIOS 2306 10/09/2014
RIP: 0010:folio_end_writeback+0x30/0x70
Code: 48 8b 07 48 89 fb 0f ba e0 12 73 0a f0 80 67 02 fb e8 d0 de 00 00 48 89 df e8 fe df ff ff 48 89 df e8 f9 ac 00 00 84 c0 75 02 <0f> 0b 48 8b 03 84 c0 79 0d be 0f 00 00 00 48 89 df e8 44 f3 ff ff
RSP: 0000:ffff8881091a3db8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffffea0004271d40 RCX: 0000000000001000
RDX: 0000000000000101 RSI: 0000000000000246 RDI: ffff888107694000
RBP: ffff888100b6f600 R08: 000000204d567e99 R09: 0000000000000200
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000001 R14: 0000000000001000 R15: ffff888107402080
FS:  00007f65ecde8940(0000) GS:ffff88840fa80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fb269292ba1 CR3: 0000000107140004 CR4: 00000000001706e0
Call Trace:
 <TASK>
 mpage_end_io+0x91/0x9b
 blk_update_request+0x200/0x2be
 scsi_end_request+0x27/0xf3
 scsi_io_completion+0x151/0x21e
 blk_complete_reqs+0x41/0x4c
 __do_softirq+0x123/0x27d
 __irq_exit_rcu+0x5a/0xcd
 common_interrupt+0x36/0xbc
 asm_common_interrupt+0x22/0x40
RIP: 0033:0x7f65edc7ab60
Code: 00 41 29 c5 4a 8d 14 ed 00 00 00 00 49 8d 34 0c 49 8d 7c 0c f8 e8 d0 5e ff ff 83 6d 20 01 eb 9f 66 2e 0f 1f 84 00 00 00 00 00 <f3> 0f 1e fa 53 48 89 fb 48 83 c7 08 e8 7f 60 ff ff 85 c0 75 0b 48
RSP: 002b:00007ffe0b70bd88 EFLAGS: 00000202
RAX: 0000000000000000 RBX: 000055ddc430c5dc RCX: 000055ddc4301f98
RDX: 00000000000000ff RSI: 000000000000000c RDI: 000055ddc4301f98
RBP: 00007ffe0b7102d0 R08: 45d54cec8b358fc3 R09: 00544145535f4449
R10: 000000000000000c R11: f17eedd8cae0d043 R12: 000055ddc4302920
R13: 000055ddc432091a R14: 000055ddc4308460 R15: 000055ddc42d90dc
 </TASK>
Modules linked in:
invalid opcode: 0000 [#2] PREEMPT SMP PTI