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
Date: Fri, 21 Apr 2023 03:25:25 +0100 [thread overview]
Message-ID: <20230421022525.GX3390869@ZenIV> (raw)
In-Reply-To: <20230421011145.GW3390869@ZenIV>
On Fri, Apr 21, 2023 at 02:11:45AM +0100, Al Viro wrote:
> 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...
OK, how about the following modification of ITER_PIPE?
* buffers might extend past the current position
* buffers (as now) are either data or zero-copy ones.
* advance past the end of data => allocate data-type buffers
and fill with zeroes.
* copying data to area prior to the end of buffers => each
zero-copy-type buffer is converted to a data one; data is copied,
but WTF else can you do? If there's a page borrowed from page
cache, we can't change its contents anyway.
* zero-copy of a page into an area overlapping the existing
buffers => treat as data copy for the overlapping part and zero-copy
the rest (if any) in normal fashion.
* iov_iter_get_pages(): convert everything involved into
data-type buffers first, then give caller references to that.
Zero-fill new pages (if any).
* iov_iter_revert() does *NOT* truncate anything
* truncation of stuff past the current position is done by
generic_file_splice_read() not only in error case, but on success
as well - starting at the end of actually read data, of course.
That would make for seekable ITER_PIPE, with zero-copy working for
normal cases. I agree that O_DIRECT would probably be better off
with ITER_BVEC.
Comments?
next prev parent reply other threads:[~2023-04-21 2:25 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-14 22:07 [PATCH v18 00/15] splice, block: Use page pinning and kill ITER_PIPE David Howells
2023-03-14 22:07 ` [PATCH v18 01/15] splice: Clean up direct_splice_read() a bit David Howells
2023-03-14 22:07 ` [PATCH v18 02/15] splice: Make do_splice_to() generic and export it David Howells
2023-03-14 22:07 ` [PATCH v18 03/15] shmem: Implement splice-read David Howells
2023-03-14 23:39 ` Linus Torvalds
2023-03-14 22:07 ` [PATCH v18 04/15] overlayfs: " David Howells
2023-03-14 22:07 ` [PATCH v18 05/15] coda: " David Howells
2023-03-14 22:07 ` [PATCH v18 06/15] tty, proc, kernfs, random: Use direct_splice_read() David Howells
2023-03-14 22:07 ` [PATCH v18 07/15] splice: Do splice read from a file without using ITER_PIPE David Howells
2023-03-14 22:07 ` [PATCH v18 08/15] cifs: Use generic_file_splice_read() David Howells
2023-03-14 22:52 ` Paulo Alcantara
2023-03-14 22:07 ` [PATCH v18 09/15] iov_iter: Kill ITER_PIPE David Howells
2023-04-20 22:22 ` Al Viro
2023-04-20 22:36 ` Al Viro
2023-04-21 1:11 ` Al Viro
2023-04-21 2:25 ` Al Viro [this message]
2023-03-14 22:07 ` [PATCH v18 10/15] iomap: Don't get an reference on ZERO_PAGE for direct I/O block zeroing David Howells
2023-03-14 22:07 ` [PATCH v18 11/15] block: Fix bio_flagged() so that gcc can better optimise it David Howells
2023-03-14 22:07 ` [PATCH v18 12/15] block: Replace BIO_NO_PAGE_REF with BIO_PAGE_REFFED with inverted logic David Howells
2023-03-14 22:07 ` [PATCH v18 13/15] block: Add BIO_PAGE_PINNED and associated infrastructure David Howells
2023-03-14 22:07 ` [PATCH v18 14/15] block: Convert bio_iov_iter_get_pages to use iov_iter_extract_pages David Howells
2023-03-14 22:07 ` [PATCH v18 15/15] block: convert bio_map_user_iov " David Howells
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