Re: [PATCH 0/8] btrfs: inode management and memory consumption improvements

[David Sterba <dsterba@suse.cz>]

On Wed, May 08, 2024 at 01:17:23PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Some inode related improvements, to use an xarray to track open inodes per
> root instead of a red black tree, reduce lock contention and use less memory
> per btrfs inode, so now we can fit 4 inodes per 4K page instead of 3.
> More details in the the change logs.

Outstanding! You managed to reduce the size by 48 bytes, on my config
from 1080 to 1032. Which unfortunately means it's still 3 inodes in a
page. The config is maximal regarding the conditional features that
affect size of struct inode. All of them could be enabled on distro
kernels (checked on openSUSE):

Ifdefs in include/linux/fs.h struct inode:

#ifdef CONFIG_FS_POSIX_ACL
#ifdef CONFIG_SECURITY
#ifdef CONFIG_CGROUP_WRITEBACK
#ifdef CONFIG_FSNOTIFY
#ifdef CONFIG_FS_ENCRYPTION
#ifdef CONFIG_FS_VERITY

There's also #ifdef __NEED_I_SIZE_ORDERED but that's for 32bit only.

This is the pahole diff summary before and after the patchset on
for-next with my reference release config:

 -       /* size: 1080, cachelines: 17, members: 39 */
 -       /* sum members: 1075, holes: 2, sum holes: 5 */
 -       /* forced alignments: 2 */
 -       /* last cacheline: 56 bytes */
 +       /* size: 1032, cachelines: 17, members: 36 */
 +       /* sum members: 1026, holes: 2, sum holes: 6 */
 +       /* forced alignments: 1 */
 +       /* last cacheline: 8 bytes */

The sum is still over 1024 so we'll need to find more tricks to reduce
the space.  There are 2 holes, one is 4 bytes (after i_otime_nsec) so
there's still some potential.