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Date:   Fri, 21 Apr 2023 02:11:45 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     David Howells <dhowells@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Christoph Hellwig <hch@infradead.org>,
        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>,
        John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH v18 09/15] iov_iter: Kill ITER_PIPE
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On Thu, Apr 20, 2023 at 11:36:57PM +0100, Al Viro wrote:

> Don't get me wrong - I'd love to kill ITER_PIPE off; it's just that tons
> of ->splice_read() duplicating the corresponding ->read_iter() up to the
> point where it would call generic_file_read_iter(), modulo ignoring
> O_DIRECT case and then calling filemap_splice_read() instead...  Painful
> and asking for trouble down the road.

FWIW, broken in this series:
	* a bunch of 9p variants (those could probably switch to direct)
	* afs
	* ceph (very likely)
	* ecryptfs (atime propagation)
	* dax on various filesystems (probably)
	* f2fs
	* gfs2
	* nfs
	* ocfs2
	* orangefs
	* xfs
	* zonefs (probably)
	* splice from UDP sockets, unless I'm misreading something.
Your sock_splice_read() still falls back to generic_file_splice_read(),
rather than to direct_splice_read() and sockets don't have O_DIRECT
in flags.  Neither do they have associated page cache ;-)

Sure, we could provide a bunch of ->splice_read(), but then we'd have to make
sure that all subsequent changes to matching ->read_iter() get duplicated -
unless they are on O_DIRECT-only paths, that is...