Re: [PATCH v3] fat: make rename recovery safe after metadata write errors

[Krystian Kaniewski <krystianmkaniewski@gmail.com>]

On 10/15/26 20:54, OGAWA Hirofumi wrote:

>> diff --git a/fs/fat/namei_msdos.c b/fs/fat/namei_msdos.c
>> index d46d1a3..0aab2b1 100644
>> --- a/fs/fat/namei_msdos.c
>> +++ b/fs/fat/namei_msdos.c
>> @@ -441,7 +441,7 @@ static int do_msdos_rename(struct inode *old_dir, unsigned char *old_name,
>>  	struct fat_slot_info old_sinfo, sinfo;
>>  	struct timespec64 ts;
>>  	loff_t new_i_pos;
>> -	int err, old_attrs, is_dir, update_dotdot, corrupt = 0;
>> +	int err, old_attrs, is_dir, update_dotdot, rollback_err = 0;
>>  
>>  	old_sinfo.bh = sinfo.bh = dotdot_bh = NULL;
>>  	old_inode = d_inode(old_dentry);
>> @@ -533,7 +533,7 @@ static int do_msdos_rename(struct inode *old_dir, unsigned char *old_name,
>>  		if (IS_DIRSYNC(new_dir)) {
>>  			err = sync_dirty_buffer(dotdot_bh);
>>  			if (err)
>> -				goto error_dotdot;
>> +				goto error_no_rollback;
>>  		}
>>  		drop_nlink(old_dir);
>>  		if (!new_inode)
>> @@ -543,7 +543,7 @@ static int do_msdos_rename(struct inode *old_dir, unsigned char *old_name,
>>  	err = fat_remove_entries(old_dir, &old_sinfo);	/* and releases bh */
>>  	old_sinfo.bh = NULL;
>>  	if (err)
>> -		goto error_dotdot;
>> +		goto error_no_rollback;
> 
> What is wrong to try to revert dotdot_bh?
> 

Thanks, you are right about dotdot_bh. After a failed dotdot write, the source entry has not been removed yet. The buffer can be read again with bh_read() and, if that succeeds, the old dotdot value can be restored. A failure while removing the old entry is different because the on-disk state of that entry may already be uncertain. I can address this in v4 patch.

>>  	inode_inc_iversion(old_dir);
>>  	fat_truncate_time(old_dir, &ts, FAT_UPDATE_CMTIME);
>>  	mark_inode_dirty(old_dir);
>> @@ -561,37 +561,26 @@ out:
>>  	brelse(old_sinfo.bh);
>>  	return err;
>>  
>> -error_dotdot:
>> -	/* data cluster is shared, serious corruption */
>> -	corrupt = 1;
>> -
>> -	if (update_dotdot) {
>> -		fat_set_start(dotdot_de, MSDOS_I(old_dir)->i_logstart);
>> -		mmb_mark_buffer_dirty(dotdot_bh,
>> -				      &MSDOS_I(old_inode)->i_metadata_bhs);
>> -		corrupt |= sync_dirty_buffer(dotdot_bh);
>> -	}
>> +error_no_rollback:
>> +	/* Do not continue an on-disk rollback that cannot be completed. */
>> +	rollback_err = err;
>>  error_inode:
>>  	fat_detach(old_inode);
>>  	fat_attach(old_inode, old_sinfo.i_pos);
>>  	MSDOS_I(old_inode)->i_attrs = old_attrs;
>>  	if (new_inode) {
>>  		fat_attach(new_inode, new_i_pos);
>> -		if (corrupt) {
>> +		if (!rollback_err) {
>>  			mark_inode_dirty(new_inode);
>> -			corrupt |= sync_inode_metadata(new_inode, 1);
>> +			rollback_err = sync_inode_metadata(new_inode, 1);
>>  		}
>> -	} else {
>> -		/*
>> -		 * If new entry was not sharing the data cluster, it
>> -		 * shouldn't be serious corruption.
>> -		 */
>> -		int err2 = fat_remove_entries(new_dir, &sinfo);
>> -		if (corrupt)
>> -			corrupt |= err2;
>> -		sinfo.bh = NULL;
>> +	} else if (!rollback_err) {
>> +		/* Read the entry block again if a failed write invalidated it. */
>> +		rollback_err = bh_read(sinfo.bh, 0);
> 
> Why do this read the bh from on disk?
> 

The bh_read(sinfo.bh, 0) is needed because a failed inode-position write clears BH_Uptodate. fat_remove_entries() modifies and dirties that retained buffer directly, which otherwise triggers the warning. Reading it first refreshes the whole block. If the read fails, the buffer is left untouched.

The same reasoning applies to both the VFAT and MS-DOS paths.

-- 
Krystian Kaniewski