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Date: Wed, 13 Dec 2023 19:21:14 +0100
From: Jan Kara <jack@suse.cz>
To: Zhang Yi <yi.zhang@huaweicloud.com>
Cc: linux-ext4@vger.kernel.org, tytso@mit.edu, adilger.kernel@dilger.ca,
	jack@suse.cz, ritesh.list@gmail.com, yi.zhang@huawei.com,
	chengzhihao1@huawei.com, yukuai3@huawei.com
Subject: Re: [RFC PATCH 3/6] ext4: correct the hole length returned by
 ext4_map_blocks()
Message-ID: <20231213182114.tzwsqpeonr5ok3j2@quack3>
References: <20231121093429.1827390-1-yi.zhang@huaweicloud.com>
 <20231121093429.1827390-4-yi.zhang@huaweicloud.com>
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In-Reply-To: <20231121093429.1827390-4-yi.zhang@huaweicloud.com>

On Tue 21-11-23 17:34:26, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> In ext4_map_blocks(), if we can't find a range of mapping in the
> extents cache, we are calling ext4_ext_map_blocks() to search the real
> path. But if the querying range was tail overlaped by a delayed extent,
> we can't find it on the real extent path, so the returned hole length
> could be larger than it really is.
> 
>       |          querying map          |
>       v                                v
>       |----------{-------------}{------|----------------}-----...
>       ^          ^             ^^                       ^
>       | uncached | hole extent ||     delayed extent    |
> 
> We have to adjust the mapping length to the next not hole extent's
> lblk before searching the extent path.
> 
> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>

So I agree the ext4_ext_determine_hole() does return a hole that does not
reflect possible delalloc extent (it doesn't even need to be straddling the
end of looked up range, does it?). But ext4_ext_put_gap_in_cache() does
actually properly trim the hole length in the status tree so I think the
problem rather is that the trimming should happen in
ext4_ext_determine_hole() instead of ext4_ext_put_gap_in_cache() and that
will also make ext4_map_blocks() return proper hole length? And then
there's no need for this special handling? Or am I missing something?

								Honza

> ---
>  fs/ext4/inode.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 4ce35f1c8b0a..94e7b8500878 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -479,6 +479,7 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>  		    struct ext4_map_blocks *map, int flags)
>  {
>  	struct extent_status es;
> +	ext4_lblk_t next;
>  	int retval;
>  	int ret = 0;
>  #ifdef ES_AGGRESSIVE_TEST
> @@ -502,8 +503,10 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>  		return -EFSCORRUPTED;
>  
>  	/* Lookup extent status tree firstly */
> -	if (!(EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY) &&
> -	    ext4_es_lookup_extent(inode, map->m_lblk, NULL, &es)) {
> +	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
> +		goto uncached;
> +
> +	if (ext4_es_lookup_extent(inode, map->m_lblk, NULL, &es)) {
>  		if (ext4_es_is_written(&es) || ext4_es_is_unwritten(&es)) {
>  			map->m_pblk = ext4_es_pblock(&es) +
>  					map->m_lblk - es.es_lblk;
> @@ -532,6 +535,23 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>  #endif
>  		goto found;
>  	}
> +	/*
> +	 * Not found, maybe a hole, need to adjust the map length before
> +	 * seraching the real extent path. It can prevent incorrect hole
> +	 * length returned if the following entries have delayed only
> +	 * ones.
> +	 */
> +	if (!(flags & EXT4_GET_BLOCKS_CREATE) && es.es_lblk > map->m_lblk) {
> +		next = es.es_lblk;
> +		if (ext4_es_is_hole(&es))
> +			next = ext4_es_skip_hole_extent(inode, map->m_lblk,
> +							map->m_len);
> +		retval = next - map->m_lblk;
> +		if (map->m_len > retval)
> +			map->m_len = retval;
> +	}
> +
> +uncached:
>  	/*
>  	 * In the query cache no-wait mode, nothing we can do more if we
>  	 * cannot find extent in the cache.
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR