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

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Message-ID: <20240613212347.GB25756@twin.jikos.cz>
Date: Thu, 13 Jun 2024 23:23:47 +0200
From: David Sterba <dsterba@...e.cz>
To: Johannes Thumshirn <Johannes.Thumshirn@....com>
Cc: "dsterba@...e.cz" <dsterba@...e.cz>,
	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 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.