Re: [PATCH 2/9] btrfs: add compressed_bio::io_sectors to trace compressed bio more elegantly

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 11/06/2021 03:31, Qu Wenruo wrote:
> For btrfs_submit_compressed_read() and btrfs_submit_compressed_write(),
> we have a pretty weird dance around compressed_bio::io_sectors:
> 
>   btrfs_submit_compressed_read/write()
>   {
> 	cb = kmalloc()
> 	refcount_set(&cb->pending_bios, 0);
> 	bio = btrfs_alloc_bio();
> 
> 	/* NOTE here, we haven't yet submitted any bio */
> 	refcount_set(&cb->pending_bios, 1);
> 
> 	for (pg_index = 0; pg_index < cb->nr_pages; pg_index++) {
> 		if (submit) {
> 			/* Here we submit bio, but we always have one
> 			 * extra pending_bios */
> 			refcount_inc(&cb->pending_bios);
> 			ret = btrfs_map_bio();
> 		}
> 	}
> 
> 	/* Submit the last bio */
> 	ret = btrfs_map_bio();
>   }
> 
> There are two reasons why we do this:
> 
> - compressed_bio::pending_bios is a refcount
>   Thus if it's reduced to 0, it can not be increased again.
> 
> - To ensure the compressed_bio is not freed by some submitted bios
>   If the submitted bio is finished before the next bio submitted,
>   we can free the compressed_bio completely.
> 
> But the above code is sometimes confusing, and we can do it better by
> just introduce a new member, compressed_bio::io_sectors.
> 
> With that member, we can easily distinguish if we're really the last
> bio at endio time, and even allows us to remove some BUG_ON() later,
> as we now know how many bytes are not yet submitted.
> 
> With this new member, now compressed_bio::pending_bios really indicates
> the pending bios, without any special handling needed.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---

This doesn't apply cleanly on current misc-next.