linux-kernel - Re: [PATCH 1/3] btrfs: rst: remove encoding field from stripe

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Date: Sun, 16 Jun 2024 20:19:08 +0200
From: David Sterba <dsterba@...e.cz>
To: Johannes Thumshirn <Johannes.Thumshirn@....com>
Cc: Johannes Thumshirn <jth@...nel.org>, Chris Mason <clm@...com>,
	Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>,
	"linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] btrfs: rst: remove encoding field from stripe_extent

On Fri, Jun 14, 2024 at 09:36:34AM +0000, Johannes Thumshirn wrote:
> On 13.06.24 23:23, David Sterba wrote:
> > On Tue, Jun 11, 2024 at 04:33:19PM +0000, Johannes Thumshirn wrote:
> >> On 11.06.24 16:37, David Sterba wrote:
> >>> On Mon, Jun 10, 2024 at 10:40:25AM +0200, Johannes Thumshirn wrote:
> >>>> -#define BTRFS_STRIPE_RAID5	5
> >>>> -#define BTRFS_STRIPE_RAID6	6
> >>>> -#define BTRFS_STRIPE_RAID1C3	7
> >>>> -#define BTRFS_STRIPE_RAID1C4	8
> >>>> -
> >>>>    struct btrfs_stripe_extent {
> >>>> -	__u8 encoding;
> >>>> -	__u8 reserved[7];
> >>>>    	/* An array of raid strides this stripe is composed of. */
> >>>> -	struct btrfs_raid_stride strides[];
> >>>> +	__DECLARE_FLEX_ARRAY(struct btrfs_raid_stride, strides);
> >>>
> >>> Is there a reason to use the __ underscore macro? I see no difference
> >>> between that and DECLARE_FLEX_ARRAY and underscore usually means that
> >>> it's special in some way.
> >>>
> >>
> >> Yes, the __ version is for UAPI, like __u8 or __le32 and so on.
> > 
> > I see, though I'd rather keep the on-disk definitions free of wrappers
> > that hide the types. We use the __ int types but that's all and quite
> > clear what it means.
> > 
> > There already are flexible members (btrfs_leaf, btrfs_node,
> > btrfs_inode_extref), using the empty[] syntax. The macro wraps the
> > distinction that c++ needs but so far the existing declarations have't
> > been problematic.  So I'd rather keep the declarations consistent.
> > 
> 
> Yes but all these examples have other members as well. After this patch, 
> btrfs_stripe_extent is a container for btrfs_raid_stride, and C doesn't 
> allow a flexmember only struct:
> 
> In file included from fs/btrfs/ctree.h:18,
>                   from fs/btrfs/delayed-inode.h:19,
>                   from fs/btrfs/super.c:32:
> ./include/uapi/linux/btrfs_tree.h:753:34: error: flexible array member 
> in a struct with no named members
>    753 |         struct btrfs_raid_stride strides[];
>        |                                  ^~~~~~~

To fix that __DECLARE_FLEX_ARRAY adds the layer of an anonymous struct
and an empty other member. We'd have to duplicate that so let's use the
macro.