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 11:45:59 +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:

>>>  	/* 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().
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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