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

[Eric Sandeen <sandeen@redhat.com>]

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@whamcloud.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@itwm.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.

-Eric

> Cheers,
> Bernd
> 
> 
> 
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