linux-ext4 - Re: [PATCH] ext4: Adjust the layout of the ext4_inode

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Message-ID: <CAHB1NagapRk62L9vn0uSKYJMMSq=9kNiB+jdaxA3JBY16f9jig@mail.gmail.com>
Date: Mon, 3 Jun 2024 22:43:16 +0800
From: JunChao Sun <sunjunchao2870@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca
Subject: Re: [PATCH] ext4: Adjust the layout of the ext4_inode_info structure
 to save memory.

Jan Kara <jack@...e.cz> 于2024年6月3日周一 22:07写道：
>
> On Mon 03-06-24 21:15:24, Junchao Sun wrote:
> > Using pahole, we can see that there are some padding holes
> > in the current ext4_inode_info structure. Adjusting the
> > layout of ext4_inode_info can reduce these holes,
> > resulting in the size of the structure decreasing
> > from 2424 bytes to 2408 bytes.
>
>
> > But AFAICT this will save two holes 4 bytes each so only 8 bytes in total?
> > Not 16?

Indeed it's 16.
Consider the layout int a; hole 0; int b; hole 1; And then move int b
to hole 0 position, this adjustment resulted in saving 8 bytes. There
are two adjustments like this, so it's 16. And GDB confirmed this.

>
> >
> > Signed-off-by: Junchao Sun <sunjunchao2870@...il.com>
>
> Otherwise looks good. Feel free to add:
>
> Reviewed-by: Jan Kara <jack@...e.cz>
>
>                                                                 Honza
>
> > ---
> >  fs/ext4/ext4.h | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 023571f8dd1b..42bcd4f749a8 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -1055,6 +1055,7 @@ struct ext4_inode_info {
> >
> >       /* Number of ongoing updates on this inode */
> >       atomic_t  i_fc_updates;
> > +     atomic_t i_unwritten; /* Nr. of inflight conversions pending */
> >
> >       /* Fast commit wait queue for this inode */
> >       wait_queue_head_t i_fc_wait;
> > @@ -1103,6 +1104,10 @@ struct ext4_inode_info {
> >
> >       /* mballoc */
> >       atomic_t i_prealloc_active;
> > +
> > +     /* allocation reservation info for delalloc */
> > +     /* In case of bigalloc, this refer to clusters rather than blocks */
> > +     unsigned int i_reserved_data_blocks;
> >       struct rb_root i_prealloc_node;
> >       rwlock_t i_prealloc_lock;
> >
> > @@ -1119,10 +1124,6 @@ struct ext4_inode_info {
> >       /* ialloc */
> >       ext4_group_t    i_last_alloc_group;
> >
> > -     /* allocation reservation info for delalloc */
> > -     /* In case of bigalloc, this refer to clusters rather than blocks */
> > -     unsigned int i_reserved_data_blocks;
> > -
> >       /* pending cluster reservations for bigalloc file systems */
> >       struct ext4_pending_tree i_pending_tree;
> >
> > @@ -1146,7 +1147,6 @@ struct ext4_inode_info {
> >        */
> >       struct list_head i_rsv_conversion_list;
> >       struct work_struct i_rsv_conversion_work;
> > -     atomic_t i_unwritten; /* Nr. of inflight conversions pending */
> >
> >       spinlock_t i_block_reservation_lock;
> >
> > --
> > 2.39.2
> >
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR