lock i_mutex for fallocate?

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

Hi All,

In ext4 punch hole, we realized that the punch hole operation needs to 
be done under i_mutex just like truncate.  i_mutex for truncate is held 
in the vfs layer, so we dont need to lock it at the file system layer, 
but vfs does not lock i_mutex for fallocate.  We can lock i_mutex for 
fallocate at the fs layer, but question was raised then: should i_mutex 
for fallocate be held in the vfs layer instead?  I do not know if other 
file systems need i_mutex to be locked for fallocate, or if they might 
be locking it already, so I am doing some investigating on this idea, 
and also the appropriate use of i_mutex in general.  Can someone provide 
some insight this topic?  Thx!

Allison Henderson