linux-ext4 - Re: [PATCH 10/13] fs: factor out a direct_write

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Date:   Mon, 22 May 2023 16:19:38 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Christoph Hellwig <hch@....de>
Cc:     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,
        "open list:F2FS FILE SYSTEM" <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 10/13] fs: factor out a direct_write_fallback helper

On Fri, 19 May 2023 at 11:36, Christoph Hellwig <hch@....de> wrote:
>
> Add a helper dealing with handling the syncing of a buffered write fallback
> for direct I/O.
>
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  fs/libfs.c         | 36 ++++++++++++++++++++++++++++
>  include/linux/fs.h |  2 ++
>  mm/filemap.c       | 59 ++++++++++------------------------------------
>  3 files changed, 50 insertions(+), 47 deletions(-)
>
> diff --git a/fs/libfs.c b/fs/libfs.c
> index 89cf614a327158..9f3791fc6e0715 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -1613,3 +1613,39 @@ u64 inode_query_iversion(struct inode *inode)
>         return cur >> I_VERSION_QUERIED_SHIFT;
>  }
>  EXPORT_SYMBOL(inode_query_iversion);
> +
> +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, end;

At this point pos will point after the end of the buffered write (as
per earlier patches), yes?

> +       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;
> +
> +       /*
> +        * We need to ensure that the page cache pages are written to disk and
> +        * invalidated to preserve the expected O_DIRECT semantics.
> +        */
> +       end = pos + buffered_written - 1;

So this calculation is wrong.

AFAICS this affects later patches as well.

Thanks,
Miklos