Re: [PATCH 09/15] btrfs: shrink the size of struct btrfs_delayed_item

[Filipe Manana <fdmanana@kernel.org>]

On Mon, Aug 22, 2022 at 03:43:26PM +0200, David Sterba wrote:
> On Wed, Aug 17, 2022 at 12:22:42PM +0100, fdmanana@kernel.org wrote:
> >  	}
> >  
> >  	item->index = index;
> > -	item->ins_or_del = BTRFS_DELAYED_DELETION_ITEM;
> >  
> >  	ret = btrfs_delayed_item_reserve_metadata(trans, item);
> >  	/*
> > diff --git a/fs/btrfs/delayed-inode.h b/fs/btrfs/delayed-inode.h
> > index fd6fe785f748..729d352ca8a1 100644
> > --- a/fs/btrfs/delayed-inode.h
> > +++ b/fs/btrfs/delayed-inode.h
> > @@ -16,9 +16,10 @@
> >  #include <linux/refcount.h>
> >  #include "ctree.h"
> >  
> > -/* types of the delayed item */
> > -#define BTRFS_DELAYED_INSERTION_ITEM	1
> > -#define BTRFS_DELAYED_DELETION_ITEM	2
> > +enum btrfs_delayed_item_type {
> > +	BTRFS_DELAYED_INSERTION_ITEM,
> > +	BTRFS_DELAYED_DELETION_ITEM
> > +};
> >  
> >  struct btrfs_delayed_root {
> >  	spinlock_t lock;
> > @@ -80,8 +81,9 @@ struct btrfs_delayed_item {
> >  	u64 bytes_reserved;
> >  	struct btrfs_delayed_node *delayed_node;
> >  	refcount_t refs;
> > -	int ins_or_del;
> > -	u32 data_len;
> > +	enum btrfs_delayed_item_type type:1;
> 
> Bit fields next to atomicly accessed variables could be problemantic on
> architectures without safe unaligned access. Either this can be :8 or
> moved after 'key' where's a 7 byte hole.

Yep, I forgot that.

The 'key' doesn't exist anymore, it was removed in a previous patch of
the series, so the 7 bytes holes is not there anymore.

I don't see any other place in the structure where I can move the fields
and still reduce its size. If switching from :1 to :8 is enough to
guarantee safety, I guess it's the only solution.

Do you me to send a new version just to switch :1 to :8 or will you do
that change?

Thanks.