linux-kernel - Re: [PATCH -next] block: fix bio lost for plug enabeld bio based device

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Date: Wed, 22 May 2024 08:38:18 +0800
From: Changhui Zhong <czhong@...hat.com>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: xni@...hat.com, axboe@...nel.dk, yukuai3@...wei.com, 
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, 
	yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH -next] block: fix bio lost for plug enabeld bio based device

On Tue, May 21, 2024 at 8:10 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
> From: Yu Kuai <yukuai3@...wei.com>
>
> With the following two conditions, bio will be lost:
>
> 1) blk plug is not enabled, for example, __blkdev_direct_IO_simple() and
> __blkdev_direct_IO_async();
> 2) bio plug is enabled, for example write IO for raid1/raid10 while
> bitmap is enabled;
>
> Root cause is that blk_finish_plug() will add the bio to
> curent->bio_list, while such bio will not be handled:
>
> __submit_bio_noacct
>  current->bio_list = bio_list_on_stack;
>  blk_start_plug
>
>  do {
>   dm_submit_bio
>    md_handle_request
>     raid10_write_request
>      -> generate new bio for underlying disks
>      raid1_add_bio_to_plug -> bio is added to plug
>  } while ((bio = bio_list_pop(&bio_list_on_stack[0])))
>  -> previous bio are all handled
>
>  blk_finish_plug
>   raid10_unplug
>    raid1_submit_write
>     submit_bio_noacct
>      if (current->bio_list)
>       bio_list_add(&current->bio_list[0], bio)
>       -> add new bio
>
>  current->bio_list = NULL
>  -> new bio is lost
>
> Fix the problem by moving plug into the while loop, so that
> current->bio_list will still be handled after blk_finish_plug().
>
> By the way, enable plug for raid1/raid10 in this case will also prevent
> delay IO handling into daemon thread, which should also improve IO
> performance.
>
> Fixes: 060406c61c7c ("block: add plug while submitting IO")
> Reported-by: Changhui Zhong <czhong@...hat.com>
> Closes: https://lore.kernel.org/all/CAGVVp+Xsmzy2G9YuEatfMT6qv1M--YdOCQ0g7z7OVmcTbBxQAg@mail.gmail.com/
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  block/blk-core.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 01186333c88e..dd29d5465af6 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -613,9 +613,14 @@ static inline blk_status_t blk_check_zone_append(struct request_queue *q,
>
>  static void __submit_bio(struct bio *bio)
>  {
> +       /* If plug is not used, add new plug here to cache nsecs time. */
> +       struct blk_plug plug;
> +
>         if (unlikely(!blk_crypto_bio_prep(&bio)))
>                 return;
>
> +       blk_start_plug(&plug);
> +
>         if (!bio->bi_bdev->bd_has_submit_bio) {
>                 blk_mq_submit_bio(bio);
>         } else if (likely(bio_queue_enter(bio) == 0)) {
> @@ -624,6 +629,8 @@ static void __submit_bio(struct bio *bio)
>                 disk->fops->submit_bio(bio);
>                 blk_queue_exit(disk->queue);
>         }
> +
> +       blk_finish_plug(&plug);
>  }
>
>  /*
> @@ -648,13 +655,11 @@ static void __submit_bio(struct bio *bio)
>  static void __submit_bio_noacct(struct bio *bio)
>  {
>         struct bio_list bio_list_on_stack[2];
> -       struct blk_plug plug;
>
>         BUG_ON(bio->bi_next);
>
>         bio_list_init(&bio_list_on_stack[0]);
>         current->bio_list = bio_list_on_stack;
> -       blk_start_plug(&plug);
>
>         do {
>                 struct request_queue *q = bdev_get_queue(bio->bi_bdev);
> @@ -688,23 +693,19 @@ static void __submit_bio_noacct(struct bio *bio)
>                 bio_list_merge(&bio_list_on_stack[0], &bio_list_on_stack[1]);
>         } while ((bio = bio_list_pop(&bio_list_on_stack[0])));
>
> -       blk_finish_plug(&plug);
>         current->bio_list = NULL;
>  }
>
>  static void __submit_bio_noacct_mq(struct bio *bio)
>  {
>         struct bio_list bio_list[2] = { };
> -       struct blk_plug plug;
>
>         current->bio_list = bio_list;
> -       blk_start_plug(&plug);
>
>         do {
>                 __submit_bio(bio);
>         } while ((bio = bio_list_pop(&bio_list[0])));
>
> -       blk_finish_plug(&plug);
>         current->bio_list = NULL;
>  }
>
> --
> 2.39.2
>

Hi， Kuai

the raid1 and raid10 issue has been fixed by this patch，
please feel free to add:

Tested-by: Changhui Zhong <czhong@...hat.com>

Thanks，