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

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Message-ID: <CAMuHMdUNoomsTKu=X5C=HBd+RAkzgcBoYyqmMy2-NsM0DqbRoQ@mail.gmail.com>
Date: Mon, 30 Sep 2024 15:08:10 +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 2:31 PM Kent Overstreet
<kent.overstreet@...ux.dev> wrote:
> On Mon, Sep 30, 2024 at 02:08:25PM GMT, Geert Uytterhoeven wrote:
> > 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.
>
> If you peruse that code even slightly, you'll see that what we're doing
> is treating the key as a multi word integer, so word order has to match
> machine byte order in order for various things in the btree lookup code
> to work.

I have seen the multi-word integers...

> But sure, try and tell me there's something about filesystems I don't
> already know...

Okay...

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