linux-kernel - Re: [PATCH v2 89/89] fs: move i_generation into new hole created after timestamp conversion

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Message-ID: <CAOQ4uxgtyaBTM1bOSSGmsk+F4ZwsK+-N5ZZ3wAt_nv_E6G3C7Q@mail.gmail.com>
Date:   Thu, 5 Oct 2023 08:08:59 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Christian Brauner <brauner@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 89/89] fs: move i_generation into new hole created
 after timestamp conversion

On Wed, Oct 4, 2023 at 9:56 PM Jeff Layton <jlayton@...nel.org> wrote:
>
> The recent change to use discrete integers instead of struct timespec64
> shaved 8 bytes off of struct inode, but it also moves the i_lock
> into the previous cacheline, away from the fields that it protects.
>
> Move i_generation above the i_lock, which moves the new 4 byte hole to
> just after the i_fsnotify_mask in my setup.

Might be good to mention that this hole has a purpose...

>
> Suggested-by: Amir Goldstein <amir73il@...il.com>
> Signed-off-by: Jeff Layton <jlayton@...nel.org>
> ---
>  include/linux/fs.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 485b5e21c8e5..686c9f33e725 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -677,6 +677,7 @@ struct inode {
>         u32                     i_atime_nsec;
>         u32                     i_mtime_nsec;
>         u32                     i_ctime_nsec;
> +       u32                     i_generation;
>         spinlock_t              i_lock; /* i_blocks, i_bytes, maybe i_size */
>         unsigned short          i_bytes;
>         u8                      i_blkbits;
> @@ -733,7 +734,6 @@ struct inode {
>                 unsigned                i_dir_seq;
>         };
>
> -       __u32                   i_generation;
>
>  #ifdef CONFIG_FSNOTIFY
>         __u32                   i_fsnotify_mask; /* all events this inode cares about */

If you post another version, please leave a comment here

+         /* 32bit hole reserved for expanding i_fsnotify_mask to 64bit */

Thanks,
Amir.