Re: [dm-devel] [RFC PATCH V2 3/3] dm: support bio polling

On 6/22/21 10:45 AM, Ming Lei wrote:

On Tue, Jun 22, 2021 at 10:26:15AM +0800, JeffleXu wrote:

quoted

On 6/21/21 10:04 PM, Ming Lei wrote:

quoted

On Mon, Jun 21, 2021 at 07:33:34PM +0800, JeffleXu wrote:

quoted

On 6/18/21 10:39 PM, Ming Lei wrote:

quoted

From 47e523b9ee988317369eaadb96826323cd86819e Mon Sep 17 00:00:00 2001
From: Ming Lei <redacted>
Date: Wed, 16 Jun 2021 16:13:46 +0800
Subject: [RFC PATCH V3 3/3] dm: support bio polling

Support bio(REQ_POLLED) polling in the following approach:

1) only support io polling on normal READ/WRITE, and other abnormal IOs
still fallback on IRQ mode, so the target io is exactly inside the dm
io.

2) hold one refcnt on io->io_count after submitting this dm bio with
REQ_POLLED

3) support dm native bio splitting, any dm io instance associated with
current bio will be added into one list which head is bio->bi_end_io
which will be recovered before ending this bio

4) implement .poll_bio() callback, call bio_poll() on the single target
bio inside the dm io which is retrieved via bio->bi_bio_drv_data; call
dec_pending() after the target io is done in .poll_bio()

4) enable QUEUE_FLAG_POLL if all underlying queues enable QUEUE_FLAG_POLL,
which is based on Jeffle's previous patch.

Signed-off-by: Ming Lei <redacted>
---
V3:
	- covers all comments from Jeffle
	- fix corner cases when polling on abnormal ios

...

One bug and one performance issue, though I haven't investigated deep
for both.


kernel base: based on Jens' for-next, applying Christoph and Leiming's
patchset.


1. One bug when there's DM device stack, e.g., dm-linear upon another
dm-linear. Can be reproduced by following steps:

$ sudo dmsetup create tmpdev --table '0 2097152 linear /dev/nvme0n1 0'

$ cat tmp.table
0 2097152 linear /dev/mapper/tmpdev 0
2097152 2097152 linear /dev/nvme0n1 0

$ cat tmp.table | dmsetup create testdev

$ fio -name=test -ioengine=io_uring -iodepth=128 -numjobs=1 -thread
-rw=randread -direct=1 -bs=4k -time_based -runtime=10 -cpus_allowed=6
-filename=/dev/mapper/testdev -hipri=1

BUG: unable to handle page fault for address: ffffffffc01a6208
#PF: supervisor write access in kernel mode
#PF: error_code(0x0003) - permissions violation
PGD 39740c067 P4D 39740c067 PUD 39740e067 PMD 1035db067 PTE 1ddf6f061
Oops: 0003 [#1] SMP PTI
CPU: 6 PID: 5899 Comm: fio Tainted: G S
5.13.0-0.1.git.81bcdc3.al7.x86_64 #1
Hardware name: Inventec     K900G3-10G/B900G3, BIOS A2.20 06/23/2017
RIP: 0010:dm_submit_bio+0x171/0x3e0 [dm_mod]

It has been fixed in my local repo:

@@ -1608,6 +1649,7 @@ static void init_clone_info(struct clone_info *ci, struct mapped_device *md,
        ci->map = map;
        ci->io = alloc_io(md, bio);
        ci->sector = bio->bi_iter.bi_sector;
+       ci->submit_as_polled = false;

It doesn't work in my test environment. Actually the following fix
should be applied.

@@ -1390,6 +1403,8 @@ static int clone_bio(struct dm_target_io *tio,

struct bio *bio,
        if (bio_integrity(bio))
                bio_integrity_trim(clone);

+       clone->bi_opf &= ~REQ_SAVED_END_IO;
+

This change is good, but it shouldn't fix the panic except for nested
device map, I will fold into V3.

The panic I posted exactly happen for nested device map.

quoted

The rationale is that, REQ_SAVED_END_IO should be cleared once the bio
*passes through* the device stack layer. Or the cloned bio for next
layer will inherit REQ_SAVED_END_IO flag, in which case
'cloned_bio->bi_end_io' (actually acts as the hlist head) won't be
initialized in dm_setup_polled_io(), and thus it gets crashed when
trying to insert into this hash list in __split_and_process_bio().

'cloned_bio' can't reach dm_submit_bio() if it isn't one DM bio.

'cloned_bio' actually refers to dm_io.tio.clone, i.e., the cloned bio
used to submit to the device of the next level.

	dm1
	/\
     dm2  NVMe1
     /\
 NVMe2 NVMe3

For the above example, 'cloned_bio' refers to dm_io.tio.clone, where
this dm_io is to be submitted to dm2.


			  @bi_private
		split bio ------------------> original bio (for dm1)
		   ^			       ^
		   | @orig_bio		       | @orig_bio
		   |			       |
		dm_io(for dm2)         	   dm_io(for NVME1)
		struct dm_target_io tio
		struct bio clone
	(...following omitted for NVMe2 and NVMe3)

I mean, for above 'struct bio clone', REQ_SAVED_END_IO shall be cleared.

-- 
Thanks,
Jeffle