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From:   David Howells <dhowells@redhat.com>
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To:     Christoph Hellwig <hch@infradead.org>
Cc:     dhowells@redhat.com, Jens Axboe <axboe@kernel.dk>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Hillf Danton <hdanton@sina.com>,
        Christian Brauner <brauner@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v20 03/32] splice: Make direct_read_splice() limit to eof where appropriate
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Date:   Fri, 19 May 2023 23:27:51 +0100
Message-ID: <2154518.1684535271@warthog.procyon.org.uk>
Precedence: bulk

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

	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 >=3D 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 i=
t.

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

David