linux-ext4 - Re: [RFC PATCH v2 05/25] ext4: make ext4_map_blocks() distinguish delalloc only extent

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Date: Wed, 3 Jan 2024 12:31:31 +0100
From: Jan Kara <jack@...e.cz>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	tytso@....edu, adilger.kernel@...ger.ca, jack@...e.cz,
	ritesh.list@...il.com, hch@...radead.org, djwong@...nel.org,
	willy@...radead.org, yi.zhang@...wei.com, chengzhihao1@...wei.com,
	yukuai3@...wei.com, wangkefeng.wang@...wei.com
Subject: Re: [RFC PATCH v2 05/25] ext4: make ext4_map_blocks() distinguish
 delalloc only extent

On Tue 02-01-24 20:38:58, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@...wei.com>
> 
> Add a new map flag EXT4_MAP_DELAYED to indicate the mapping range is a
> delayed allocated only (not unwritten) one, and making
> ext4_map_blocks() can distinguish it, no longer mixing it with holes.
> 
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>

One small comment below.

> ---
>  fs/ext4/ext4.h    | 4 +++-
>  fs/ext4/extents.c | 5 +++--
>  fs/ext4/inode.c   | 2 ++
>  3 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index a5d784872303..55195909d32f 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -252,8 +252,10 @@ struct ext4_allocation_request {
>  #define EXT4_MAP_MAPPED		BIT(BH_Mapped)
>  #define EXT4_MAP_UNWRITTEN	BIT(BH_Unwritten)
>  #define EXT4_MAP_BOUNDARY	BIT(BH_Boundary)
> +#define EXT4_MAP_DELAYED	BIT(BH_Delay)
>  #define EXT4_MAP_FLAGS		(EXT4_MAP_NEW | EXT4_MAP_MAPPED |\
> -				 EXT4_MAP_UNWRITTEN | EXT4_MAP_BOUNDARY)
> +				 EXT4_MAP_UNWRITTEN | EXT4_MAP_BOUNDARY |\
> +				 EXT4_MAP_DELAYED)
>  
>  struct ext4_map_blocks {
>  	ext4_fsblk_t m_pblk;
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 0892d0568013..fc69f13cf510 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -4073,9 +4073,10 @@ static void ext4_ext_determine_hole(struct inode *inode,
>  	} else if (in_range(map->m_lblk, es.es_lblk, es.es_len)) {
>  		/*
>  		 * Straddle the beginning of the queried range, it's no
> -		 * longer a hole, adjust the length to the delayed extent's
> -		 * after map->m_lblk.
> +		 * longer a hole, mark it is a delalloc and adjust the
> +		 * length to the delayed extent's after map->m_lblk.
>  		 */
> +		map->m_flags |= EXT4_MAP_DELAYED;

I wouldn't set delalloc bit here. If there's delalloc extent containing
lblk now, it means the caller of ext4_map_blocks() is not holding i_rwsem
(otherwise we would have found already in ext4_map_blocks()) and thus
delalloc info is unreliable anyway. So I wouldn't bother. But it's worth a
comment here like:

		/*
		 * There's delalloc extent containing lblk. It must have
		 * been added after ext4_map_blocks() checked the extent
		 * status tree so we are not holding i_rwsem and delalloc
		 * info is only stabilized by i_data_sem we are going to
		 * release soon. Don't modify the extent status tree and
		 * report extent as a hole.
		 */

								Honza

>  		len = es.es_lblk + es.es_len - map->m_lblk;
>  		goto out;
>  	} else {
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 1b5e6409f958..c141bf6d8db2 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -515,6 +515,8 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>  			map->m_len = retval;
>  		} else if (ext4_es_is_delayed(&es) || ext4_es_is_hole(&es)) {
>  			map->m_pblk = 0;
> +			map->m_flags |= ext4_es_is_delayed(&es) ?
> +					EXT4_MAP_DELAYED : 0;
>  			retval = es.es_len - (map->m_lblk - es.es_lblk);
>  			if (retval > map->m_len)
>  				retval = map->m_len;
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR