linux-kernel - Re: [PATCH] bcachefs: rename version -> bversion for big endian builds

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Message-ID: <CAMuHMdXZ_N+uvXROpNvvSO3AZ7A-7hQTE4FxEa=1aMX-NC5rbA@mail.gmail.com>
Date: Mon, 30 Sep 2024 14:08:25 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Guenter Roeck <linux@...ck-us.net>, linux-bcachefs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] bcachefs: rename version -> bversion for big endian builds

Hi Kent,

On Mon, Sep 30, 2024 at 12:11 PM Kent Overstreet
<kent.overstreet@...ux.dev> wrote:
> On Mon, Sep 30, 2024 at 12:04:42PM GMT, Geert Uytterhoeven wrote:
> > On Mon, Sep 30, 2024 at 2:39 AM Guenter Roeck <linux@...ck-us.net> wrote:
> > > Builds on big endian systems fail as follows.
> > >
> > > fs/bcachefs/bkey.h: In function 'bch2_bkey_format_add_key':
> > > fs/bcachefs/bkey.h:557:41: error:
> > >         'const struct bkey' has no member named 'bversion'
> > >
> > > The original commit only renamed the variable for little endian builds.
> > > Rename it for big endian builds as well to fix the problem.
> > >
> > > Fixes: cf49f8a8c277 ("bcachefs: rename version -> bversion")
> >
> > Which is (again) not found on any mailing list, and has never been in
> > linux-next before it hit upstream...
> >
> > > Cc: Kent Overstreet <kent.overstreet@...ux.dev>
> > > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> >
> > > --- a/fs/bcachefs/bcachefs_format.h
> > > +++ b/fs/bcachefs/bcachefs_format.h
> > > @@ -223,7 +223,7 @@ struct bkey {
> > >  #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> > >         struct bpos     p;
> > >         __u32           size;           /* extent size, in sectors */
> > > -       struct bversion version;
> > > +       struct bversion bversion;
> > >
> > >         __u8            pad[1];
> > >  #endif
> >
> > BTW, how does this work when accessing a non-native file system?
> > Didn't we stop doing bi-endian file systems in v2.1.10, when ext2 was
> > converted from a bi-endian to a little-endian file system?
>
> we byte swab if necessary

So you have to test 4 combinations instead of 2 (which you don't do,
obviously ;-)

Ext2 was converted from a bi-endian to a little-endian file system
because it turned out the conditional byte-swapping was more
expensive than unconditional (not) byte-swapping. Given all the
bcache structures are already tagged with __packed anyway, I guess
this is even more true for bcachefs.

The proper way established +25y ago was to settle on one endianness
layout for all on-disk data. That way you do not have to duplicate
data and code for little vs. big endian, keep both paths in sync, and
you can annotate everything with __[bl]eXX attributes to let sparse
help you catch bugs.

Which endianness to pick is up to you. Ext2 settled on little-endian,
XFS on big-endian.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds