Re: [PATCH v3 5/6] fat: permit to return phy block number by fibmapin fallocated region

[Namjae Jeon]

2014-02-04, OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>:
> Namjae Jeon <linkinjeon@xxxxxxxxx> writes:
>
>>>>  	/* fat_get_cluster() assumes the requested blocknr isn't truncated.
>>>> */
>>>>  	down_read(&MSDOS_I(mapping->host)->truncate_lock);
>>>> +	/* To get block number beyond file size in fallocated region */
>>>> +	atomic_set(&MSDOS_I(mapping->host)->beyond_isize, 1);
>>>>  	blocknr = generic_block_bmap(mapping, block, fat_get_block);
>>>> +	atomic_set(&MSDOS_I(mapping->host)->beyond_isize, 0);
>>>>  	up_read(&MSDOS_I(mapping->host)->truncate_lock);
>>>
>>> This is racy. While user is using bmap, kernel can allocate new blocks.
>>> We should use another function for this.
>> I understand that fat can map fallocated blocks in read case while
>> user is using bmap.
>> But I can not find the case allocate new blocks.
>> If I am missing something, Could you please elaborate more ?
>> Is it a case of _bmap request returning the block number for block
>> allocated in parallel write path ?
>
> ->beyond_size is global for inode. So, write(2) path on same inode with
> bmap() also can see 1 set by bmap() while another process is using bmap().
'create' flag  will be 1 in write(2) path. ->beyond_isize will only be
checked when 'create' flag is 0. Is there any case to be racy by
beyond_isize in write(2) path ?

Thanks.
> --
> OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
>
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