linux-ext4 - Re: [PATCH 09/11] fs: factor out a direct_write

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Date:   Thu, 25 May 2023 12:05:57 +0200
From:   Christoph Hellwig <hch@....de>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Xiubo Li <xiubli@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Damien Le Moal <dlemoal@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-block@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 09/11] fs: factor out a direct_write_fallback helper

On Wed, May 24, 2023 at 09:00:36AM +0200, Miklos Szeredi wrote:
> > +ssize_t direct_write_fallback(struct kiocb *iocb, struct iov_iter *iter,
> > +               ssize_t direct_written, ssize_t buffered_written)
> > +{
> > +       struct address_space *mapping = iocb->ki_filp->f_mapping;
> > +       loff_t pos = iocb->ki_pos - buffered_written;
> > +       loff_t end = iocb->ki_pos - 1;
> > +       int err;
> > +
> > +       /*
> > +        * If the buffered write fallback returned an error, we want to return
> > +        * the number of bytes which were written by direct I/O, or the error
> > +        * code if that was zero.
> > +        *
> > +        * Note that this differs from normal direct-io semantics, which will
> > +        * return -EFOO even if some bytes were written.
> > +        */
> > +       if (unlikely(buffered_written < 0))
> > +               return buffered_written;
> 
> Comment/code mismatch.   The comment says:
> 
> if (buffered_written < 0)
>         return direct_written ?: buffered_written;

Yeah.  And the old code matches the comment, so I'll update to that.
I'm really wondering how I could come up with a good test case for
this..