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

[Chi Zhiling <chizhiling@163.com>]

On 11/28/25 10:48, Sungjong Seo wrote:
> 
> 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.

Hi, Sungjong

I agree.

My original plan was to support multiple clusters for exfat_map_cluster 
and exfat_get_cluster. since the changes required were quite extensive, 
I put that plan on hold. This would likely involve refactoring 
exfat_map_clusterand introducing iterators to reduce the number of 
parameters it needs

I will take some time to consider the signature of the new 
exfat_map_clusters. Do you have any thoughts about this?

> 
>>
>> 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?

This is not the expected behavior. It's a serious bug. Thank you very 
much for pointing this out.

> 
>> +		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);