Re: [PATCH] fs/address_space: move i_mmap_rwsem to mitigate a false sharing with i_mmap.

[Matthew Wilcox]

On Fri, Feb 02, 2024 at 05:34:07PM +0800, JonasZhou-oc wrote:
> In the struct address_space, there is a 32-byte gap between i_mmap
> and i_mmap_rwsem. Due to the alignment of struct address_space
> variables to 8 bytes, in certain situations, i_mmap and
> i_mmap_rwsem may end up in the same CACHE line.
> 
> While running Unixbench/execl, we observe high false sharing issues
> when accessing i_mmap against i_mmap_rwsem. We move i_mmap_rwsem
> after i_private_list, ensuring a 64-byte gap between i_mmap and
> i_mmap_rwsem.

I'm confused.  i_mmap_rwsem protects i_mmap.  Usually you want the lock
and the thing it's protecting in the same cacheline.  Why is that not
the case here?