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

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Message-ID: <ZGhEj5RhZYwCnzKG@infradead.org>
Date:   Fri, 19 May 2023 20:54:55 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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

On Fri, May 19, 2023 at 11:27:51PM +0100, David Howells wrote:
> 	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().

The zero length check makes sense in vfs_splice_read.  The ppos check
I think makes sense for filemap_splice_read - after all we're dealing
with the page cache here, and the page cache needs a working maxbytes
and i_size.  What callers of filemap_splice_read that don't have the
checks do you have in your tree right now and how did you end up with
them?