linux-ext4 - Re: [PATCH] ext4: delayed allocation i

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Date:	Tue, 08 Jul 2008 08:15:34 -0400
From:	Peter Staubach <staubach@...hat.com>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	Mingming Cao <cmm@...ibm.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: delayed allocation i_blocks fix for stat(2)

Eric Sandeen wrote:
> Mingming Cao wrote:
>   
>> ext4: delayed allocation i_blocks fix for stat(2)
>>
>> From: Mingming Cao <cmm@...ibm.com>
>>
>> Right now i_blocks is not getting updated until the disks are actually
>> allocaed on disk.  This means with delayed allocation, right after files
>> are copied, "ls -sF" shoes the file as taking 0 blocks on disk.  "du"
>> also shows the files taking zero space, which is highly confusing to the user.
>>
>> Since current delayed allocation already keep track of per-inode total number
>> of blocks that are subject to delayed allocation, this patch fix this by using
>> that to adjust the value returned by stat(2). When real block allocation
>> is done, the i_blocks will get updated. Since the reserved blocks for delayed
>> allocation will be decreased, this will be keep value returned by stat(2)
>> consistent.
>>
>> Signed-off-by: Mingming Cao <cmm@...ibm.com>
>>     
>
> Thanks Mingming, looks like just the right approach.
>
> Something about the spinlock for every stat seems heavy-handed to me but
> I'll have to give that more thought.  :)
>
>   

Since i_reserved_blocks is an unsigned long, it should be possible
to atomically fetch it on all of the supported architectures,
without the use of the spinlock.  It seems to me that this spinlock
is not required here.

       ps

> -Eric
>
>   
>> ---
>>  fs/ext4/ext4.h  |    2 ++
>>  fs/ext4/file.c  |    1 +
>>  fs/ext4/inode.c |   26 ++++++++++++++++++++++++++
>>  3 files changed, 29 insertions(+)
>>
>> Index: linux-2.6.26-rc9/fs/ext4/ext4.h
>> ===================================================================
>> --- linux-2.6.26-rc9.orig/fs/ext4/ext4.h	2008-07-07 16:29:23.000000000 -0700
>> +++ linux-2.6.26-rc9/fs/ext4/ext4.h	2008-07-07 16:32:54.000000000 -0700
>> @@ -1059,6 +1059,8 @@ int ext4_get_blocks_handle(handle_t *han
>>  extern struct inode *ext4_iget(struct super_block *, unsigned long);
>>  extern int  ext4_write_inode (struct inode *, int);
>>  extern int  ext4_setattr (struct dentry *, struct iattr *);
>> +extern int  ext4_getattr(struct vfsmount *mnt, struct dentry *dentry,
>> +				struct kstat *stat);
>>  extern void ext4_delete_inode (struct inode *);
>>  extern int  ext4_sync_inode (handle_t *, struct inode *);
>>  extern void ext4_discard_reservation (struct inode *);
>> Index: linux-2.6.26-rc9/fs/ext4/file.c
>> ===================================================================
>> --- linux-2.6.26-rc9.orig/fs/ext4/file.c	2008-07-07 16:29:21.000000000 -0700
>> +++ linux-2.6.26-rc9/fs/ext4/file.c	2008-07-07 16:29:51.000000000 -0700
>> @@ -161,6 +161,7 @@ const struct file_operations ext4_file_o
>>  const struct inode_operations ext4_file_inode_operations = {
>>  	.truncate	= ext4_truncate,
>>  	.setattr	= ext4_setattr,
>> +	.getattr	= ext4_getattr,
>>  #ifdef CONFIG_EXT4DEV_FS_XATTR
>>  	.setxattr	= generic_setxattr,
>>  	.getxattr	= generic_getxattr,
>> Index: linux-2.6.26-rc9/fs/ext4/inode.c
>> ===================================================================
>> --- linux-2.6.26-rc9.orig/fs/ext4/inode.c	2008-07-07 16:29:22.000000000 -0700
>> +++ linux-2.6.26-rc9/fs/ext4/inode.c	2008-07-07 16:31:13.000000000 -0700
>> @@ -4203,6 +4203,32 @@ err_out:
>>  	return error;
>>  }
>>  
>> +int ext4_getattr(struct vfsmount *mnt, struct dentry *dentry,
>> +		 struct kstat *stat)
>> +{
>> +	struct inode *inode;
>> +	unsigned long delalloc_blocks;
>> +
>> +	inode = dentry->d_inode;
>> +	generic_fillattr(inode, stat);
>> +
>> +	/*
>> +	 * We can't update i_blocks if the block allocation is delayed
>> +	 * otherwise in the case of system crash before the real block
>> +	 * allocation is done, we will have i_blocks inconsistent with
>> +	 * on-disk file blocks.
>> +	 * We always keep i_blocks updated together with real
>> +	 * allocation. But to not confuse with user, stat
>> +	 * will return the blocks that include the delayed allocation
>> +	 * blocks for this file.
>> +	 */
>> +	spin_lock(&EXT4_I(inode)->i_block_reservation_lock);
>> +	delalloc_blocks = EXT4_I(inode)->i_reserved_data_blocks;
>> +	spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
>> +
>> +	stat->blocks += (delalloc_blocks << inode->i_sb->s_blocksize_bits)>>9;
>> +	return 0;
>> +}
>>  
>>  /*
>>   * How many blocks doth make a writepage()?
>>
>>
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>>     
>
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>   

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