Re: i_mutex questions

[Allison Henderson <achender@linux.vnet.ibm.com>]

On 09/13/2011 09:23 PM, Andreas Dilger wrote:
> On 2011-09-13, at 7:29 PM, Yongqiang Yang wrote:
>> On Wed, Sep 14, 2011 at 2:33 AM, Allison Henderson
>> <achender@linux.vnet.ibm.com>  wrote:
>>> Hi All,
>>>
>>> I have been trying to find a way to synchronize punch hole with read and
>>> write operations with out the use of i_mutex.  The concern is that after
>>> punch hole has released the pages inside the hole, another process may remap
>>> the page to a block before punch has taken i_data_sem.  I think putting
>>> i_mutex around the punch hole operation would fix this, but since we are
>>> trying to avoid further improper use of i_mutex, I am trying to avoid that
>>> solution.
>>>
>>> I cannot use i_data_sem to protect the pages because it seems most of the
>>> code has already established a locking order of pages first, then
>>> i_data_sem.  So moving i_data_sem up tends to cause a lot of dead locks.
>>>   I'm thinking that there probably needs to be a another mutex involved some
>>> where, but I wasnt sure if some one is already working on the idea of
>>> introducing a replacement for i_mutex.  So I just wanted to know if there
>>> are any plans already in motion for this, or if any one else could suggest
>>> some ideas for the punch hole issue.  Thx all!
>>
>> Lukas sent out a patch ([PATCH] ext4: Make reads/writes atomic with
>> i_rwlock semaphore) which collected some feedbacks suggesting using
>> extent lock instead of a read-write semaphore.  If there is extent
>> lock implementation in ext4, then fallocate can use it, maybe
>> dioread-nolock can use it as well, e.g. locking a range and unlocking
>> the range until the extent is converted from unwritten to init.
>
> We have a prototype patch for extent locking for ext4.  We are planning to
> use this for parallel locking of htree directories, but it could potentially
> be modified to for extent locking of files.
>
> The current patch is below, but it hasn't gone through a lot of testing:
>
> http://review.whamcloud.com/#patch,sidebyside,375,2,ldiskfs/kernel_patches/patches/ext4_pdirop-rhel6.patch
>
> Cheers, Andreas
>

Alrighty then, I will have a look at the existing patches.  Thx!

Allison Henderson

>
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