linux-ext4 - Re: [PATCH] ext4: Don't set s_buddy_cache->i_ino to EXT4_BAD

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Date:	Wed, 17 Aug 2011 23:38:51 +0800
From:	Tao Ma <tm@....ma>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Yu Jian <yujian@...mcloud.com>,
	Andreas Dilger <adilger@...mcloud.com>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH] ext4: Don't set s_buddy_cache->i_ino to EXT4_BAD_INO

On 08/17/2011 11:31 PM, Andreas Dilger wrote:
> On 2011-08-17, at 2:20 AM, Tao Ma <tm@....ma> wrote:
>> In 85fe4025, Christoph removed get_next_ino() from new_inode() and
>> do s_buddy_cache->i_ino = get_next_ino();
>> And then 48e6061b gives EXT4_BAD_INO to this inode. But actually
>> s_buddy_cache is used directly and we never get it from an inode number.
>> So it should be safe for us to not set i_ino at all and I guess that
>> is the case Christoph described in his commit log of 85fe4025
>> "For a few more filesystems we can avoid assigning any inode number given
>> that they aren't user visible".
> 
> I don't see that setting i_ino is harmful here (only done once per filesystem mount) and the comment clearly indicates that it is for precautionary reasons only. 
> 
> If this inode happens to appear in a crashdump for some reason, I'd prefer it has an inode number that can be identified easily rather than some random value left over from a previously allocated and freed inode, because inode_init_always() does not zero out i_ino. 
Fair enough. Thanks for the explanation.

Thanks
Tao
> 
>> Cc: Yu Jian <yujian@...mcloud.com>
>> Cc: Andreas Dilger <adilger@...mcloud.com>
>> Cc: "Theodore Ts'o" <tytso@....edu>
>> Signed-off-by: Tao Ma <boyu.mt@...bao.com>
>> ---
>> fs/ext4/mballoc.c |    5 -----
>> 1 files changed, 0 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>> index 17a5a57..982f783 100644
>> --- a/fs/ext4/mballoc.c
>> +++ b/fs/ext4/mballoc.c
>> @@ -2342,11 +2342,6 @@ static int ext4_mb_init_backend(struct super_block *sb)
>>        ext4_msg(sb, KERN_ERR, "can't get new inode");
>>        goto err_freesgi;
>>    }
>> -    /* To avoid potentially colliding with an valid on-disk inode number,
>> -     * use EXT4_BAD_INO for the buddy cache inode number.  This inode is
>> -     * not in the inode hash, so it should never be found by iget(), but
>> -     * this will avoid confusion if it ever shows up during debugging. */
>> -    sbi->s_buddy_cache->i_ino = EXT4_BAD_INO;
>>    EXT4_I(sbi->s_buddy_cache)->i_disksize = 0;
>>    for (i = 0; i < ngroups; i++) {
>>        desc = ext4_get_group_desc(sb, i, NULL);
>> -- 
>> 1.7.0.4
>>
>> --
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