Plan for reducing i_mutex in ext4

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

Hi all,

I've been working on locating all the existing uses of i_mutex in the 
current ext4 code because I know we are planning to reduce the usage of 
i_mutex in ext4.  So I've gone through the ext4 code and also the vfs 
code and come up with a list of ext4 items that appear to be protected 
under i_mutex.  I'm thinking about doing a patch to replace i_mutex with 
a private ext4 mutex, and I wanted to update folks on this idea and pick 
up any feed back people might have.

I'm thinking maybe we can have a separate mutex for functions that only 
modify meta data like ext4_ioctl and ext4_setattr to help relieve 
unneeded contention.  And then the rest of functions that are modifying 
data can go under a data mutex (including truncate since sometimes 
ext4_ioctl and ext4_setattr will call ext4_truncate if they modify i_size).

So these are ext4 functions that currently lock i_mutex:

ext4_sync_file
ext4_fallocate
ext4_move_extents via two helper routines:
	mext_inode_double_lock and mext_inode_double_unlock
ext4_ioctl (for the EXT4_IOC_SETFLAGS ioctl)
ext4_quota_write
ext4_llseek
ext4_end_io_work
ext4_evict_inode (only while calling ext4_flush_completed_IO)
ext4_ind_direct_IO (only while calling ext4_flush_completed_IO)


And these are ext4 functions that have i_mutex locked by the vfs layer. 
  So we will need to lock the new private mutex here too if we want them 
to be synchronous with the above functions.

ext4_setattr
ext4_da_writepages
ext4_rmdir
ext4_unlink
ext4_symlink
ext4_link
ext4_rename

And one unique case:
ext4_fiemap calls generic_block_fiemap and passes it a function pointer 
to ext4_get_block.  generic_block_fiemap will lock i_mutex before 
calling the pointer.  I dont think ext4_get_block needs i_mutex locked 
all the time, so I think we can just make a wrapper for ext4_get_block 
that locks the new private mutex and then we can pass a pointer to the 
wrapper.


That's my list so far, if anyone knows of one I missed please let me 
know, and also if you spot any other places where we can reduce unneeded 
contention by using a separate lock.  Thx!

Allison Henderson