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Subject: Re: [PATCH] btrfs: block-group: Fix free-space bitmap threshould
To:     Marcos Paulo de Souza <marcos@mpdesouza.com>,
        linux-kernel@vger.kernel.org
Cc:     dsterba@suse.com, wqu@suse.com, linux-btrfs@vger.kernel.org,
        Marcos Paulo de Souza <mpdesouza@suse.com>,
        stable@vger.kernel.org
References: <20200821024231.16256-1-marcos@mpdesouza.com>
From:   Qu Wenruo <quwenruo.btrfs@gmx.com>
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Message-ID: <9953a211-5068-c31d-3cf7-fe8441f3c38c@gmx.com>
Date:   Fri, 21 Aug 2020 11:34:30 +0800
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On 2020/8/21 =E4=B8=8A=E5=8D=8810:42, Marcos Paulo de Souza wrote:
> From: Marcos Paulo de Souza <mpdesouza@suse.com>
>=20
> [BUG]
> After commit 9afc66498a0b ("btrfs: block-group: refactor how we read on=
e
> block group item"), cache->length is being assigned after calling
> btrfs_create_block_group_cache. This causes a problem since
> set_free_space_tree_thresholds is calculate the free-space threshould t=
o
> decide is the free-space tree should convert from extents to bitmaps.
>=20
> The current code calls set_free_space_tree_thresholds with cache->lengt=
h
> being 0, which then makes cache->bitmap_high_thresh being zero. This
> implies the system will always use bitmap instead of extents, which is
> not desired if the block group is not fragmented.
>=20
> This behavior can be seen by a test that expects to repair systems
> with FREE_SPACE_EXTENT and FREE_SPACE_BITMAP, but the current code only=

> created FREE_SPACE_BITMAP.
>=20
> [FIX]
> Call set_free_space_tree_thresholds after setting cache->length.
>=20
> Link: https://github.com/kdave/btrfs-progs/issues/251
> Fixes: 9afc66498a0b ("btrfs: block-group: refactor how we read one bloc=
k group item")
> CC: stable@vger.kernel.org # 5.8+
> Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

It would be even nicer if you could add some warning or self-test on
cache->length to prevent such problem from happening again.

Thanks,
Qu
> ---
>  fs/btrfs/block-group.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>=20
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index 44fdfa2eeb2e..01e8ba1da1d3 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -1798,7 +1798,6 @@ static struct btrfs_block_group *btrfs_create_blo=
ck_group_cache(
> =20
>  	cache->fs_info =3D fs_info;
>  	cache->full_stripe_len =3D btrfs_full_stripe_len(fs_info, start);
> -	set_free_space_tree_thresholds(cache);
> =20
>  	cache->discard_index =3D BTRFS_DISCARD_INDEX_UNUSED;
> =20
> @@ -1908,6 +1907,8 @@ static int read_one_block_group(struct btrfs_fs_i=
nfo *info,
> =20
>  	read_block_group_item(cache, path, key);
> =20
> +	set_free_space_tree_thresholds(cache);
> +
>  	if (need_clear) {
>  		/*
>  		 * When we mount with old space cache, we need to
> @@ -2128,6 +2129,7 @@ int btrfs_make_block_group(struct btrfs_trans_han=
dle *trans, u64 bytes_used,
>  		return -ENOMEM;
> =20
>  	cache->length =3D size;
> +	set_free_space_tree_thresholds(cache);
>  	cache->used =3D bytes_used;
>  	cache->flags =3D type;
>  	cache->last_byte_to_unpin =3D (u64)-1;
>=20


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