linux-kernel - Re: [PATCH v2 1/3] splice: always fsnotify_access(in), fsnotify

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Message-ID: <CAOQ4uxjis1CbC+ZMXrr3ez4b=X4PRSWE6NVN=vFgukJOjuGPqQ@mail.gmail.com>
Date:   Tue, 27 Jun 2023 09:02:12 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Ahelenia Ziemiańska 
        <nabijaczleweli@...ijaczleweli.xyz>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>,
        Chung-Chiang Cheng <cccheng@...ology.com>
Subject: Re: [PATCH v2 1/3] splice: always fsnotify_access(in),
 fsnotify_modify(out) on success

On Tue, Jun 27, 2023 at 2:09 AM Ahelenia Ziemiańska
<nabijaczleweli@...ijaczleweli.xyz> wrote:
>
> The current behaviour caused an asymmetry where some write APIs
> (write, sendfile) would notify the written-to/read-from objects,
> but splice wouldn't.
>
> This affected userspace which used inotify, like coreutils tail -f.
>

typo: uses?

But this comment is not very clear IMO.
Please imagine that the reader is a distro maintainer or coreutils maintainer
How are they supposed to react to this comment?
Is this an important fix for them to backport??
How does this change actually affect tail -f?
Does it fix a bug in tail -f?
As far as I understand from our last conversation, the answer is
that it does not fix a bug in tail -f and it won't affect tail -f at all -
it would *allow* tail -f to be changed in a way that could improve
some use cases. Right? improve in what way exactly.

The simplest way to explain this fix perhaps would be to link to
a patch to tail and explain how the kernel+tail fixes improve a
use case.


> Fixes: 983652c69199 ("splice: report related fsnotify events")
> Link: https://lore.kernel.org/linux-fsdevel/jbyihkyk5dtaohdwjyivambb2gffyjs3dodpofafnkkunxq7bu@jngkdxx65pux/t/#u
> Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@...ijaczleweli.xyz>
> Reviewed-by: Amir Goldstein <amir73il@...il.com>
> ---
> No changes since v1 (except in the message).
>
>  fs/splice.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/fs/splice.c b/fs/splice.c
> index 3e06611d19ae..94fae24f9d54 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -1154,7 +1154,8 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
>                 if ((in->f_flags | out->f_flags) & O_NONBLOCK)
>                         flags |= SPLICE_F_NONBLOCK;
>
> -               return splice_pipe_to_pipe(ipipe, opipe, len, flags);
> +               ret = splice_pipe_to_pipe(ipipe, opipe, len, flags);
> +               goto notify;
>         }
>
>         if (ipipe) {
> @@ -1182,15 +1183,12 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
>                 ret = do_splice_from(ipipe, out, &offset, len, flags);
>                 file_end_write(out);
>
> -               if (ret > 0)
> -                       fsnotify_modify(out);
> -
>                 if (!off_out)
>                         out->f_pos = offset;
>                 else
>                         *off_out = offset;
>
> -               return ret;
> +               goto notify;
>         }
>
>         if (opipe) {
> @@ -1209,18 +1207,23 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
>
>                 ret = splice_file_to_pipe(in, opipe, &offset, len, flags);
>
> -               if (ret > 0)
> -                       fsnotify_access(in);
> -
>                 if (!off_in)
>                         in->f_pos = offset;
>                 else
>                         *off_in = offset;
>
> -               return ret;
> +               goto notify;
>         }
>
>         return -EINVAL;
> +
> +notify:
> +       if (ret > 0) {
> +               fsnotify_access(in);
> +               fsnotify_modify(out);
> +       }
> +
> +       return ret;
>  }
>

Sorry I haven't noticed this in the first review, but goto is not really needed.
We make the three cases if{}else if{}else if{}
and return -EINVAL in the else case.

It's not really that important, just a bit nicer IMO, so as you wish.

Thanks,
Amir.