Re: [RFC PATCH 7/7] exfat: get mutil-clusters in exfat_get_block

[Sungjong Seo <sj1557.seo@samsung.com>]

Hi, Chi,
On 25. 11. 18. 17:22, Chi Zhiling wrote:
> From: Chi Zhiling <chizhiling@kylinos.cn>
> 
> mpage uses the get_block of the file system to obtain the mapping of a
> file or allocate blocks for writes. Currently exfat only supports
> obtaining one cluster in each get_block call.
> 
> Since exfat_count_contig_clusters can obtain multiple consecutive clusters,
> it can be used to improve exfat_get_block when page size is larger than
> cluster size.

I think reusing buffer_head is a good approach!
However, for obtaining multiple clusters, it would be better to handle
them in exfat_map_cluster.

> 
> Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
> ---
>  fs/exfat/inode.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/exfat/inode.c b/fs/exfat/inode.c
> index f9501c3a3666..256ba2af34eb 100644
> --- a/fs/exfat/inode.c
> +++ b/fs/exfat/inode.c
> @@ -264,13 +264,14 @@ static int exfat_map_cluster(struct inode *inode, unsigned int clu_offset,
>  static int exfat_get_block(struct inode *inode, sector_t iblock,
>  		struct buffer_head *bh_result, int create)
>  {
> +	struct exfat_chain chain;
>  	struct exfat_inode_info *ei = EXFAT_I(inode);
>  	struct super_block *sb = inode->i_sb;
>  	struct exfat_sb_info *sbi = EXFAT_SB(sb);
>  	unsigned long max_blocks = bh_result->b_size >> inode->i_blkbits;
>  	int err = 0;
>  	unsigned long mapped_blocks = 0;
> -	unsigned int cluster, sec_offset;
> +	unsigned int cluster, sec_offset, count;
>  	sector_t last_block;
>  	sector_t phys = 0;
>  	sector_t valid_blks;
> @@ -301,6 +302,17 @@ static int exfat_get_block(struct inode *inode, sector_t iblock,
>  
>  	phys = exfat_cluster_to_sector(sbi, cluster) + sec_offset;
>  	mapped_blocks = sbi->sect_per_clus - sec_offset;
> +
> +	if (max_blocks > mapped_blocks && !create) {
> +		chain.dir = cluster;
> +		chain.size = (max_blocks >> sbi->sect_per_clus_bits) + 1;

There seems to be an issue where the code sets chain.size to be one greater than the actual cluster count.

For example, assuming a 16KiB page, 512B sector, and 4KiB cluster,
for a 16KiB file, chain.size becomes 5 instead of 4.
Is this the intended behavior?

> +		chain.flags = ei->flags;
> +
> +		err = exfat_count_contig_clusters(sb, &chain, &count);
> +		if (err)
> +			return err;
> +		max_blocks = (count << sbi->sect_per_clus_bits) - sec_offset;

You already said mapped_blocks is correct.

> +	}
>  	max_blocks = min(mapped_blocks, max_blocks);
>  
>  	map_bh(bh_result, sb, phys);