linux-ext4 - Re: [PATCH RFC 5/8] ext4: use super write guard in write_mmp

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Message-ID: <CAPjX3FdKdV5ouW3Vjx1jMO8Ye_21x5CDtpOn+BBD_tou1gPkSg@mail.gmail.com>
Date: Thu, 6 Nov 2025 11:04:22 +0100
From: Daniel Vacek <neelx@...e.com>
To: Jan Kara <jack@...e.cz>
Cc: Christian Brauner <brauner@...nel.org>, linux-fsdevel@...r.kernel.org, 
	Alexander Viro <viro@...iv.linux.org.uk>, linux-btrfs@...r.kernel.org, 
	linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH RFC 5/8] ext4: use super write guard in write_mmp_block()

On Thu, 6 Nov 2025 at 10:24, Jan Kara <jack@...e.cz> wrote:
>
> On Wed 05-11-25 19:33:35, Daniel Vacek wrote:
> > On Tue, 4 Nov 2025 at 13:16, Christian Brauner <brauner@...nel.org> wrote:
> > >
> > > Signed-off-by: Christian Brauner <brauner@...nel.org>
> > > ---
> > >  fs/ext4/mmp.c | 8 ++------
> > >  1 file changed, 2 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
> > > index ab1ff51302fb..6f57c181ff77 100644
> > > --- a/fs/ext4/mmp.c
> > > +++ b/fs/ext4/mmp.c
> > > @@ -57,16 +57,12 @@ static int write_mmp_block_thawed(struct super_block *sb,
> > >
> > >  static int write_mmp_block(struct super_block *sb, struct buffer_head *bh)
> > >  {
> > > -       int err;
> > > -
> > >         /*
> > >          * We protect against freezing so that we don't create dirty buffers
> > >          * on frozen filesystem.
> > >          */
> > > -       sb_start_write(sb);
> > > -       err = write_mmp_block_thawed(sb, bh);
> > > -       sb_end_write(sb);
> > > -       return err;
> > > +       scoped_guard(super_write, sb)
> > > +               return write_mmp_block_thawed(sb, bh);
> >
> > Why the scoped_guard here? Should the simple guard(super_write)(sb) be
> > just as fine here?
>
> Not sure about Ted but I prefer scoped_guard() to plain guard() because the
> scoping makes it more visually obvious where the unlocking happens. Of
> course there has to be a balance as the indentation level can go through
> the roof but that's not the case here...

In a general case I completely agree. Though here you can see the end
of the block right on the next line so it looks quite obvious to me
how far the guarded region spans.

But the question is whether to use the guards at all. To me it's a
huge change in reading and understanding the code as more things are
implied and not explicit. Such implied things result in additional
cognitive load. I guess we can handle it eventually. Yet we can fail
from time to time and it is likely we will, at least in the beginning.

Well, my point is this is a new style we're not used to, yet. It just
takes time to adapt. (And usually a few failures to do so.)

--nX

>                                                                 Honza
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR