Re: [PATCH v9 18/41] btrfs: reset zones of unused block groups

From: Josef Bacik <josef@toxicpanda.com>
To: Naohiro Aota <naohiro.aota@wdc.com>,
	linux-btrfs@vger.kernel.org, dsterba@suse.com
Cc: hare@suse.com, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v9 18/41] btrfs: reset zones of unused block groups
Date: Tue, 3 Nov 2020 09:34:19 -0500	[thread overview]
Message-ID: <7b48d9f1-53d8-2526-e628-13331e4fe344@toxicpanda.com> (raw)
In-Reply-To: <575e495d534c44aded9e6ae042a9d6bda5c84162.1604065695.git.naohiro.aota@wdc.com>

On 10/30/20 9:51 AM, Naohiro Aota wrote:
> For an ZONED volume, a block group maps to a zone of the device. For
> deleted unused block groups, the zone of the block group can be reset to
> rewind the zone write pointer at the start of the zone.
> 
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
> ---
>   fs/btrfs/block-group.c |  8 ++++++--
>   fs/btrfs/extent-tree.c | 17 ++++++++++++-----
>   fs/btrfs/zoned.h       | 16 ++++++++++++++++
>   3 files changed, 34 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index d67f9cabe5c1..82d556368c85 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -1468,8 +1468,12 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
>   		if (!async_trim_enabled && btrfs_test_opt(fs_info, DISCARD_ASYNC))
>   			goto flip_async;
>   
> -		/* DISCARD can flip during remount */
> -		trimming = btrfs_test_opt(fs_info, DISCARD_SYNC);
> +		/*
> +		 * DISCARD can flip during remount. In ZONED mode, we need
> +		 * to reset sequential required zones.
> +		 */
> +		trimming = btrfs_test_opt(fs_info, DISCARD_SYNC) ||
> +				btrfs_is_zoned(fs_info);
>   
>   		/* Implicit trim during transaction commit. */
>   		if (trimming)
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 5e6b4d1712f2..c134746d7417 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -1331,6 +1331,9 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
>   
>   		stripe = bbio->stripes;
>   		for (i = 0; i < bbio->num_stripes; i++, stripe++) {
> +			struct btrfs_device *dev = stripe->dev;
> +			u64 physical = stripe->physical;
> +			u64 length = stripe->length;
>   			u64 bytes;
>   			struct request_queue *req_q;
>   
> @@ -1338,14 +1341,18 @@ int btrfs_discard_extent(struct btrfs_fs_info *fs_info, u64 bytenr,
>   				ASSERT(btrfs_test_opt(fs_info, DEGRADED));
>   				continue;
>   			}
> +
>   			req_q = bdev_get_queue(stripe->dev->bdev);
> -			if (!blk_queue_discard(req_q))
> +			/* zone reset in ZONED mode */
> +			if (btrfs_can_zone_reset(dev, physical, length))
> +				ret = btrfs_reset_device_zone(dev, physical,
> +							      length, &bytes);
> +			else if (blk_queue_discard(req_q))
> +				ret = btrfs_issue_discard(dev->bdev, physical,
> +							  length, &bytes);
> +			else
>   				continue;
>   
> -			ret = btrfs_issue_discard(stripe->dev->bdev,
> -						  stripe->physical,
> -						  stripe->length,
> -						  &bytes);

The problem is you have

if (btrfs_test_opt(fs_info, DISCARD_SYNC))
	ret = btrfs_discard_extent(fs_info, start,
				   end + 1 - start, NULL);

in btrfs_finish_extent_commit, so even if you add support here, you aren't 
actually discarding anything because the transaction commit won't call discard 
for this range.

You're going to have to rework this logic to allow for discard to be called 
everywhere it checks DISCARD_SYNC, but then you also need to go and make sure 
you don't actually allow discards to happen at non-bg aligned ranges.  Thanks,

Josef