Re: [dm-devel] [PATCH 1/3] dm: don't grab target io reference in dm_zone_map_bio

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 4/11/22 23:18, Ming Lei wrote:
>>>> This fixes the issue:
>>>>
>>>> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
>>>> index 3c5fad7c4ee6..3dd6735450c5 100644
>>>> --- a/drivers/md/dm.c
>>>> +++ b/drivers/md/dm.c
>>>> @@ -581,7 +581,7 @@ static struct dm_io *alloc_io(struct mapped_device
>>>> *md, struct bio *bio)
>>>>         io->status = 0;
>>>>         atomic_set(&io->io_count, 1);
>>>>         this_cpu_inc(*md->pending_io);
>>>> -       io->orig_bio = NULL;
>>>> +       io->orig_bio = bio;
>>>>         io->md = md;
>>>>         io->map_task = current;
>>>>         spin_lock_init(&io->lock);
>>>>
>>>> Otherwise, the dm-zone.c code sees a NULL orig_bio.
>>>> However, this change may be messing up the bio accounting. Need to check that.
>>>
>>> Looks it is one recent regression since:
>>>
>>> commit 0fbb4d93b38b ("dm: add dm_submit_bio_remap interface")
>>
>> Yep, saw that. Problem is, I really do not understand that change setting
>> io->orig_bio *after* __map_bio() is called. It seems that the accounting
>> is done on each fragment of the orig_bio instead of once for the entire
>> BIO... So my "fix" above seems wrong. Apart from passing along orig_bio as
>> an argument to  __map_bio() from __split_and_process_bio(), I do not think
>> my change is correct. Thoughts ?
> 
> Frankly speaking, both changing ->orig_bio after split and setting ->orig_bio
> after ->map() looks ugly & tricky, and the following change should avoid the
> issue, meantime simplify dm accounting a bit:
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 3c5fad7c4ee6..f1fe83113608 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -581,7 +581,7 @@ static struct dm_io *alloc_io(struct mapped_device *md, struct bio *bio)
>  	io->status = 0;
>  	atomic_set(&io->io_count, 1);
>  	this_cpu_inc(*md->pending_io);
> -	io->orig_bio = NULL;
> +	io->orig_bio = bio;
>  	io->md = md;
>  	io->map_task = current;
>  	spin_lock_init(&io->lock);
> @@ -1223,19 +1223,11 @@ void dm_submit_bio_remap(struct bio *clone, struct bio *tgt_clone)
>  	 * Account io->origin_bio to DM dev on behalf of target
>  	 * that took ownership of IO with DM_MAPIO_SUBMITTED.
>  	 */
> -	if (io->map_task == current) {
> +	if (io->map_task == current)
>  		/* Still in target's map function */
>  		dm_io_set_flag(io, DM_IO_START_ACCT);
> -	} else {
> -		/*
> -		 * Called by another thread, managed by DM target,
> -		 * wait for dm_split_and_process_bio() to store
> -		 * io->orig_bio
> -		 */
> -		while (unlikely(!smp_load_acquire(&io->orig_bio)))
> -			msleep(1);
> +	else

Curly brackets around the else here.

>  		dm_start_io_acct(io, clone);
> -	}
>  
>  	__dm_submit_bio_remap(tgt_clone, disk_devt(io->md->disk),
>  			      tio->old_sector);
> @@ -1562,7 +1554,7 @@ static void dm_split_and_process_bio(struct mapped_device *md,
>  				     struct dm_table *map, struct bio *bio)
>  {
>  	struct clone_info ci;
> -	struct bio *orig_bio = NULL;
> +	struct bio *new_bio = NULL;
>  	int error = 0;
>  
>  	init_clone_info(&ci, md, map, bio);
> @@ -1578,22 +1570,14 @@ static void dm_split_and_process_bio(struct mapped_device *md,
>  	if (error || !ci.sector_count)
>  		goto out;
>  
> -	/*
> -	 * Remainder must be passed to submit_bio_noacct() so it gets handled
> -	 * *after* bios already submitted have been completely processed.
> -	 * We take a clone of the original to store in ci.io->orig_bio to be
> -	 * used by dm_end_io_acct() and for dm_io_complete() to use for
> -	 * completion handling.
> -	 */

This comment should remain with some adjustment.

> -	orig_bio = bio_split(bio, bio_sectors(bio) - ci.sector_count,
> -			     GFP_NOIO, &md->queue->bio_split);
> -	bio_chain(orig_bio, bio);
> -	trace_block_split(orig_bio, bio->bi_iter.bi_sector);
> -	submit_bio_noacct(bio);
> +	new_bio = bio_alloc_clone(bio->bi_bdev, bio, GFP_NOIO,
> +			&md->queue->bio_split);

Why not keep using bio_split() ?

> +	bio_trim(new_bio, bio_sectors(bio) - ci.sector_count, ci.sector_count);
> +	bio_trim(bio, 0, bio_sectors(bio) - ci.sector_count);
> +	bio_chain(new_bio, bio);
> +	trace_block_split(new_bio, new_bio->bi_iter.bi_sector);
> +	submit_bio_noacct(new_bio);
>  out:
> -	if (!orig_bio)
> -		orig_bio = bio;
> -	smp_store_release(&ci.io->orig_bio, orig_bio);
>  	if (dm_io_flagged(ci.io, DM_IO_START_ACCT))
>  		dm_start_io_acct(ci.io, NULL);

I tested this and it works. Need to check the accounting though.
And I agree this is a lot cleaner :)

-- 
Damien Le Moal
Western Digital Research

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