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, 04 Feb 2014 15:55:37 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Namjae Jeon <linkinjeon@...il.com>
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

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.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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