Re: Two questions regarding ext4_fallocate()

[Ji Wu <wu_ji2012@163.com>]

Hi Theodore,
      Thanks for your explanation.
      These questions are originally raised by my friend, after
a discussion, we did not figure out an exact answer. Now
I think I can ask him to prepare patch for it. Actually, we did find
this useless call applies to some other file systems.

Cheers,
Ji Wu

On 05/05/2013 01:33 AM, Theodore Ts'o wrote:
> On Sat, May 04, 2013 at 10:58:50PM +0800, Ji Wu wrote:
>> Hi,
>>     I have two questions regarding ext4_fallocate(),
>>
>>     (1) The first is the FALLOC_FL_PUNCH_HOLE support, I am wondering
>> what is the usage for it? The only use case comes to my mind is
>> while ext4 being used for virtual machine image file storage. When
>> VMM is aware of the file deleting operation in guest os, it can
>> invoke host file system's fallocate() on the virtual machine image
>> file to punch a hole to free host storage, so that save host
>> space. But how can VMM being aware of guest file deleting? Simulate
>> a virtual SSD-like block device to guest os, then capture the TRIM
>> instruction issued by guest file system? That seems too tricky.  So
>> basically, where and how to benefit from hole punching?
> It's not too tricky; all of the hypervisors, whether it's KVM, or Xen,
> or VMWare, are already simulating a SATA device to the guest OS.
> Implementing support for the TRIM request is not that hard, and most
> of the hypervisors are doing this already.  Implementing the punch
> hole functionality was indeed primarily motivated for this use case.
>
> The other historical use of this was for digital video recorders, but
> that's a much more specialized use case.
>
>>     (2) At the beginning of the function ext4_ext_punch_hole(), the
>> codes are as follows,
>>
>>          /* write out all dirty pages to avoid race condition */
>>          filemap_write_and_wait_range(mapping, offset, offset+length-1);
>>          mutex_lock(&inode->i_mutex);
>>          truncate_page_cache_range();
>>
>>      Why does it need synchronously write back the dirty pages fit
>> into the hole, the data on the disk responding to those pages are to
>> be deleted, why not directly release those pages, no matter they are
>> dirty or not.  And furthermore, this is done before the inode lock is
>> held, so it seems it may happen that after the pages are written
>> back, and before the lock is held, those pages are dirtied again.
>> So basically, why does it need call filemap_write_and_wait_range()
>> before releasing those pages?
> That's a good a question.  Looking at it, I'm not sure we do.  I
> suspect this was put in originally to avoid races with setting the
> EOFBLOCKS_FL flag, but as you point out, there's no way we can prevent
> writes to sneak in before we grab the i_mutex.  As a result, we ended
> up dropping the need for EOFBLOCKS_FL entirely.
>
> Maybe one of the ext4 developers will see something that I'm missing,
> but I think we can drop this, which indeed will have a significant
> performance improvement for systems that use the punch hole
> functionality.
>
> Cheers,
>
> 						- Ted
>