linux-kernel - Re: [PATCH v3 5/6] fat: permit to return phy block number by fibmap in fallocated region

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Date:	Tue, 4 Feb 2014 16:00:17 +0900
From:	Namjae Jeon <linkinjeon@...il.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Namjae Jeon <namjae.jeon@...sung.com>,
	Amit Sahrawat <a.sahrawat@...sung.com>
Subject: Re: [PATCH v3 5/6] fat: permit to return phy block number by fibmap
 in fallocated region

2014-02-04, OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>:
> Namjae Jeon <linkinjeon@...il.com> writes:
>
>> 2014-02-04, OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>:
>>> Namjae Jeon <linkinjeon@...il.com> 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 ?
>
> Ah, so instead of write, it will assign physical address to buffers
> beyond i_size for simple read if race?  In this case, it is still wrong.
Right. I will fix this case.
Thanks for review!
> --
> OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
>
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