linux-kernel - Re: [PATCH v20 03/32] splice: Make direct_read

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Message-ID: <CAHk-=whX+mAESz01NJZssoLMsgEpFjx7LDLO1_uW1qaDY2Jidw@mail.gmail.com>
Date:   Fri, 19 May 2023 09:31:17 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v20 03/32] splice: Make direct_read_splice() limit to eof
 where appropriate

On Fri, May 19, 2023 at 12:41 AM David Howells <dhowells@...hat.com> wrote:
>
> +
> +       if (S_ISREG(file_inode(in)->i_mode) ||
> +           S_ISBLK(file_inode(in)->i_mode)) {

This really feels fundamentally wrong to me.

If block and regular files have this limit, they should have their own
splice_read() function that implements that limit.

Not make everybody else check it.

IOW, this should be a separate function ("block_splice_read()" or
whatever), not inside a generic function that other users use.

The zero size checking looks fine, although I wondered about that too.
Some special files do traditionally have special meanings for
zero-sized reads (as in "packet boundary"). But I suspect that isn't
an issue for splice, and perhaps more importantly, I think the same
rule should be in place: special files that want special rules
shouldn't be using this generic function directly then.

                 Linus