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

[David Sterba <dsterba@suse.cz>]

On Mon, Aug 22, 2022 at 03:15:32PM +0100, Filipe Manana wrote:
> 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.

Right, I did not check on the exact commit first.

> 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.

Yeah that's find and there's only one byte unused, leaving the char
data[] aligned to 8 bytes:

struct btrfs_delayed_item {
        struct rb_node             rb_node __attribute__((__aligned__(8))); /*     0    24 */
        u64                        index;                /*    24     8 */
        struct list_head           tree_list;            /*    32    16 */
        struct list_head           readdir_list;         /*    48    16 */
        /* --- cacheline 1 boundary (64 bytes) --- */
        u64                        bytes_reserved;       /*    64     8 */
        struct btrfs_delayed_node * delayed_node;        /*    72     8 */
        refcount_t                 refs;                 /*    80     4 */
        enum btrfs_delayed_item_type type:8;             /*    84: 0  4 */

        /* XXX 8 bits hole, try to pack */
        /* Bitfield combined with next fields */

        u16                        data_len;             /*    86     2 */
        char                       data[];               /*    88     0 */

        /* size: 88, cachelines: 2, members: 10 */
        /* sum members: 86 */
        /* sum bitfield members: 8 bits, bit holes: 1, sum bit holes: 8 bits */
        /* forced alignments: 1 */
        /* last cacheline: 24 bytes */
} __attribute__((__aligned__(8)));

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

I'll do that, it's a simple change but I wanted to let you know about
that.