linux-ext4 - Re: [PATCH 5 2/4] Return 32/64-bit dir name hash according to usage type

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Date:	Mon, 23 Apr 2012 16:22:47 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
CC:	linux-nfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Fan Yong <yong.fan@...mcloud.com>,
	bfields@...hat.com, Andreas Dilger <adilger@...mcloud.com>
Subject: Re: [PATCH 5 2/4] Return 32/64-bit dir name hash according to usage
 type

On 4/23/12 3:52 PM, Bernd Schubert wrote:
> On 04/23/2012 10:37 PM, Eric Sandeen wrote:
>> On 4/22/12 7:51 AM, Bernd Schubert wrote:
>>> On 04/20/2012 10:04 PM, Eric Sandeen wrote:
>>>> On 1/9/12 7:21 AM, Bernd Schubert wrote:
>>>>> From: Fan Yong <yong.fan@...mcloud.com>
>>>>>
>>>>> Traditionally ext2/3/4 has returned a 32-bit hash value from llseek()
>>>>> to appease NFSv2, which can only handle a 32-bit cookie for seekdir()
>>>>> and telldir().  However, this causes problems if there are 32-bit hash
>>>>> collisions, since the NFSv2 server can get stuck resending the same
>>>>> entries from the directory repeatedly.
>>>>>
>>>>> Allow ext4 to return a full 64-bit hash (both major and minor) for
>>>>> telldir to decrease the chance of hash collisions.  This still needs
>>>>> integration on the NFS side.
>>>>>
>>>>> Patch-updated-by: Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
>>>>> (blame me if something is not correct)
>>>>
>>>> Bernd, I've merged this to ext3.  Bruce thought maybe you were working
>>>> on the same.  Should I send mine?
>>>
>>> That is perfectly fine with me.
>>>
>>>>
>>>> Also...
>>>>
>>>>> +/*
>>>>> + * ext4_dir_llseek() based on generic_file_llseek() to handle both
>>>>> + * non-htree and htree directories, where the "offset" is in terms
>>>>> + * of the filename hash value instead of the byte offset.
>>>>> + *
>>>>> + * NOTE: offsets obtained *before* ext4_set_inode_flag(dir, EXT4_INODE_INDEX)
>>>>> + *       will be invalid once the directory was converted into a dx directory
>>>>> + */
>>>>> +loff_t ext4_dir_llseek(struct file *file, loff_t offset, int origin)
>>>>
>>>> ext4_llseek() worries about max offset for direct/indirect vs. extent-mapped
>>>> files.  Do we need to worry about the same thing in this function?
>>>
>>> Hrmm, I just checked it and I think either is wrong. We only have to
>>> care about non-dx directories, so ext4_readdir() applies, which limits
>>> filp->f_pos < inode->i_size.
>>> Going to send a patch tomorrow. Thanks for spotting this!
>>
>> The other thing I'm wondering is whether, in light of
>>
>> ef3d0fd27e90f67e35da516dafc1482c82939a60 vfs: do (nearly) lockless generic_file_llseek
>>
>> taking the i_mutex in ext4_dir_llseek could be a perf regression vs what was there before?  Is there anything about the new function which requires stronger locking?
>>
>> I may be missing something obvious about the nfs interaction, not sure.
>>
> 
> Oh, good point. I was just about to send a small patch, but reading
> through the lockless commit will take some time - its already too late
> for me for today. Will work on that tomorrow. Thanks again for your review!

Sorry it's so late :(

-Eric

> Cheers,
> Bernd
> 
> 
> 
> diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
> index b867862..3a4988e2 100644
> --- a/fs/ext4/dir.c
> +++ b/fs/ext4/dir.c
> @@ -363,7 +363,7 @@ loff_t ext4_dir_llseek(struct file *file, loff_t
> offset, int origin)
>  		goto out_err;
> 
>  	if (!dx_dir) {
> -		if (offset > inode->i_sb->s_maxbytes)
> +		if (offset > i_size_read(inode))
>  			goto out_err;
>  	} else if (offset > ext4_get_htree_eof(file))
>  		goto out_err;
> 
> 

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