linux-kernel - Re: [RFC PATCH 7/7] exfat: get mutil-clusters in exfat_get

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Message-ID: <a19281dc-4b8a-4a86-a2e4-64da2a499015@163.com>
Date: Fri, 5 Dec 2025 19:36:32 +0800
From: Chi Zhiling <chizhiling@....com>
To: Sungjong Seo <sj1557.seo@...sung.com>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Matthew Wilcox <willy@...radead.org>, Namjae Jeon <linkinjeon@...nel.org>,
 Yuezhang Mo <yuezhang.mo@...y.com>, Chi Zhiling <chizhiling@...inos.cn>
Subject: Re: [RFC PATCH 7/7] exfat: get mutil-clusters in exfat_get_block

On 12/4/25 20:18, Sungjong Seo wrote:
> 
> 
> On 25. 11. 28. 15:18, Chi Zhiling wrote:
>> On 11/28/25 10:48, Sungjong Seo wrote:
>>>
>>> Hi, Chi,
>>> On 25. 11. 18. 17:22, Chi Zhiling wrote:
>>>> From: Chi Zhiling <chizhiling@...inos.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?
> Apologies, I missed your email.
> IMO, we don't need to rush, so I think expanding exfat_map_cluster(s) would be better.

Okay.

> 
> Thanks.
>>
>>>
>>>>
>>>> Signed-off-by: Chi Zhiling <chizhiling@...inos.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);
>>
>>