Plan for reducing i_mutex in ext4

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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
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