linux-kernel - Re: [PATCH v3 2/3] btrfs: Split remaining space to discard in chunks

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Message-ID: <20240903193936.GK26776@twin.jikos.cz>
Date: Tue, 3 Sep 2024 21:39:36 +0200
From: David Sterba <dsterba@...e.cz>
To: Luca Stefani <luca.stefani.ge1@...il.com>
Cc: Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
	David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] btrfs: Split remaining space to discard in chunks

On Tue, Sep 03, 2024 at 09:16:11AM +0200, Luca Stefani wrote:
> Per Qu Wenruo in case we have a very large disk, e.g. 8TiB device,
> mostly empty although we will do the split according to our super block
> locations, the last super block ends at 256G, we can submit a huge
> discard for the range [256G, 8T), causing a super large delay.
> 
> We now split the space left to discard based the block discard limit
> in preparation of introduction of cancellation signals handling.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=219180
> Link: https://bugzilla.suse.com/show_bug.cgi?id=1229737
> Signed-off-by: Luca Stefani <luca.stefani.ge1@...il.com>
> ---
>  fs/btrfs/extent-tree.c | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index a5966324607d..9c1ddf13659e 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -1301,12 +1301,26 @@ static int btrfs_issue_discard(struct block_device *bdev, u64 start, u64 len,
>  	}
>  
>  	if (bytes_left) {
> -		ret = blkdev_issue_discard(bdev, start >> SECTOR_SHIFT,
> -					   bytes_left >> SECTOR_SHIFT,
> -					   GFP_NOFS);
> -		if (!ret)
> -			*discarded_bytes += bytes_left;
> +		u64 bytes_to_discard;
> +		struct bio *bio = NULL;
> +		sector_t sector = start >> SECTOR_SHIFT;
> +		sector_t nr_sects = bytes_left >> SECTOR_SHIFT;
> +
> +		while ((bio = blk_alloc_discard_bio(bdev, &sector, &nr_sects,
> +				GFP_NOFS))) {
> +			ret = submit_bio_wait(bio);
> +			bio_put(bio);
> +
> +			if (!ret)
> +				bytes_to_discard = bio->bi_iter.bi_size;
> +			else if (ret != -EOPNOTSUPP)
> +				return ret;
> +
> +			start += bytes_to_discard;
> +			bytes_left -= bytes_to_discard;
> +		}

This is not what I anticipated, we only wanted to know the optimal
request size but now it's reimplementing the bio submission and compared
to blkdev_issue_discard() it lacks blk_start_plug/blk_finish_plug.

As we won't get the bio_discard_limit() export for some reason I suggest
to go back to setting the maximum chunk limit in our code and set it to
something like 8G.