linux-ext4 - Re: [RFC PATCH v2 02/25] ext4: convert to exclusive lock while inserting delalloc extents

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Message-ID: <20240103100325.jmzlai2xegn4skwm@quack3>
Date: Wed, 3 Jan 2024 11:03:25 +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 02/25] ext4: convert to exclusive lock while
 inserting delalloc extents

On Tue 02-01-24 20:38:55, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@...wei.com>
> 
> ext4_da_map_blocks() only hold i_data_sem in shared mode and i_rwsem
> when inserting delalloc extents, it could be raced by another querying
> path of ext4_map_blocks() without i_rwsem, .e.g buffered read path.
> Suppose we buffered read a file containing just a hole, and without any
> cached extents tree, then it is raced by another delayed buffered write
> to the same area or the near area belongs to the same hole, and the new
> delalloc extent could be overwritten to a hole extent.
> 
>  pread()                           pwrite()
>   filemap_read_folio()
>    ext4_mpage_readpages()
>     ext4_map_blocks()
>      down_read(i_data_sem)
>      ext4_ext_determine_hole()
>      //find hole
>      ext4_ext_put_gap_in_cache()
>       ext4_es_find_extent_range()
>       //no delalloc extent
>                                     ext4_da_map_blocks()
>                                      down_read(i_data_sem)
>                                      ext4_insert_delayed_block()
>                                      //insert delalloc extent
>       ext4_es_insert_extent()
>       //overwrite delalloc extent to hole
> 
> This race could lead to inconsistent delalloc extents tree and
> incorrect reserved space counter. Fix this by converting to hold
> i_data_sem in exclusive mode when adding a new delalloc extent in
> ext4_da_map_blocks().
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> Suggested-by: Jan Kara <jack@...e.cz>

Looks good to me! Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/ext4/inode.c | 25 +++++++++++--------------
>  1 file changed, 11 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 5b0d3075be12..142c67f5c7fc 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1703,10 +1703,8 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
>  
>  	/* Lookup extent status tree firstly */
>  	if (ext4_es_lookup_extent(inode, iblock, NULL, &es)) {
> -		if (ext4_es_is_hole(&es)) {
> -			down_read(&EXT4_I(inode)->i_data_sem);
> +		if (ext4_es_is_hole(&es))
>  			goto add_delayed;
> -		}
>  
>  		/*
>  		 * Delayed extent could be allocated by fallocate.
> @@ -1748,8 +1746,10 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
>  		retval = ext4_ext_map_blocks(NULL, inode, map, 0);
>  	else
>  		retval = ext4_ind_map_blocks(NULL, inode, map, 0);
> -	if (retval < 0)
> -		goto out_unlock;
> +	if (retval < 0) {
> +		up_read(&EXT4_I(inode)->i_data_sem);
> +		return retval;
> +	}
>  	if (retval > 0) {
>  		unsigned int status;
>  
> @@ -1765,24 +1765,21 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
>  				EXTENT_STATUS_UNWRITTEN : EXTENT_STATUS_WRITTEN;
>  		ext4_es_insert_extent(inode, map->m_lblk, map->m_len,
>  				      map->m_pblk, status);
> -		goto out_unlock;
> +		up_read(&EXT4_I(inode)->i_data_sem);
> +		return retval;
>  	}
> +	up_read(&EXT4_I(inode)->i_data_sem);
>  
>  add_delayed:
> -	/*
> -	 * XXX: __block_prepare_write() unmaps passed block,
> -	 * is it OK?
> -	 */
> +	down_write(&EXT4_I(inode)->i_data_sem);
>  	retval = ext4_insert_delayed_block(inode, map->m_lblk);
> +	up_write(&EXT4_I(inode)->i_data_sem);
>  	if (retval)
> -		goto out_unlock;
> +		return retval;
>  
>  	map_bh(bh, inode->i_sb, invalid_block);
>  	set_buffer_new(bh);
>  	set_buffer_delay(bh);
> -
> -out_unlock:
> -	up_read((&EXT4_I(inode)->i_data_sem));
>  	return retval;
>  }
>  
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR