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

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Message-ID: <2154518.1684535271@warthog.procyon.org.uk>
Date:   Fri, 19 May 2023 23:27:51 +0100
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     dhowells@...hat.com, Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        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>,
        Linus Torvalds <torvalds@...ux-foundation.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

Okay.  Let's go with that.  So I have to put the handling in vfs_splice_read():

	long vfs_splice_read(struct file *in, loff_t *ppos,
			     struct pipe_inode_info *pipe, size_t len,
			     unsigned int flags)
	{
	...
		if (unlikely(!in->f_op->splice_read))
			return warn_unsupported(in, "read");
		/*
		 * O_DIRECT and DAX don't deal with the pagecache, so we
		 * allocate a buffer, copy into it and splice that into the pipe.
		 */
		if ((in->f_flags & O_DIRECT) || IS_DAX(in->f_mapping->host))
			return copy_splice_read(in, ppos, pipe, len, flags);
		return in->f_op->splice_read(in, ppos, pipe, len, flags);
	}

which leaves very little in generic_file_splice_read:

	ssize_t generic_file_splice_read(struct file *in, loff_t *ppos,
					 struct pipe_inode_info *pipe, size_t len,
					 unsigned int flags)
	{
		if (unlikely(*ppos >= in->f_mapping->host->i_sb->s_maxbytes))
			return 0;
		if (unlikely(!len))
			return 0;
		return filemap_splice_read(in, ppos, pipe, len, flags);
	}

so I wonder if the tests in generic_file_splice_read() can be folded into
vfs_splice_read(), pointers to generic_file_splice_read() be replaced with
pointers to filemap_splice_read() and generic_file_splice_read() just be
removed.

I suspect we can't quite do this because of the *ppos check - but I wonder if
that's actually necessary since filemap_splice_read() checks against
i_size... or if the check can be moved there if we definitely want to do it.

Certainly, the zero-length check can be done in vfs_splice_read().

David