From: David Hildenbrand <david@redhat.com>
To: David Howells <dhowells@redhat.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Christoph Hellwig <hch@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>,
Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
Jeff Layton <jlayton@kernel.org>,
Logan Gunthorpe <logang@deltatee.com>,
linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
John Hubbard <jhubbard@nvidia.com>,
linux-mm@kvack.org
Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator
Date: Mon, 23 Jan 2023 12:28:40 +0100 [thread overview]
Message-ID: <246ba813-698b-8696-7f4d-400034a3380b@redhat.com> (raw)
In-Reply-To: <20230120175556.3556978-3-dhowells@redhat.com>
On 20.01.23 18:55, David Howells wrote:
> Add a function, iov_iter_extract_pages(), to extract a list of pages from
> an iterator. The pages may be returned with a reference added or a pin
> added or neither, depending on the type of iterator and the direction of
> transfer. The caller must pass FOLL_READ_FROM_MEM or FOLL_WRITE_TO_MEM
> as part of gup_flags to indicate how the iterator contents are to be used.
>
> Add a second function, iov_iter_extract_mode(), to determine how the
> cleanup should be done.
>
> There are three cases:
>
> (1) Transfer *into* an ITER_IOVEC or ITER_UBUF iterator.
>
> Extracted pages will have pins obtained on them (but not references)
> so that fork() doesn't CoW the pages incorrectly whilst the I/O is in
> progress.
>
> iov_iter_extract_mode() will return FOLL_PIN for this case. The
> caller should use something like unpin_user_page() to dispose of the
> page.
>
> (2) Transfer is *out of* an ITER_IOVEC or ITER_UBUF iterator.
>
> Extracted pages will have references obtained on them, but not pins.
>
> iov_iter_extract_mode() will return FOLL_GET. The caller should use
> something like put_page() for page disposal.
>
> (3) Any other sort of iterator.
>
> No refs or pins are obtained on the page, the assumption is made that
> the caller will manage page retention. ITER_ALLOW_P2PDMA is not
> permitted.
>
> iov_iter_extract_mode() will return 0. The pages don't need
> additional disposal.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Al Viro <viro@zeniv.linux.org.uk>
> cc: Christoph Hellwig <hch@lst.de>
> cc: John Hubbard <jhubbard@nvidia.com>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: linux-fsdevel@vger.kernel.org
> cc: linux-mm@kvack.org
> Link: https://lore.kernel.org/r/166920903885.1461876.692029808682876184.stgit@warthog.procyon.org.uk/ # v2
> Link: https://lore.kernel.org/r/166997421646.9475.14837976344157464997.stgit@warthog.procyon.org.uk/ # v3
> Link: https://lore.kernel.org/r/167305163883.1521586.10777155475378874823.stgit@warthog.procyon.org.uk/ # v4
> Link: https://lore.kernel.org/r/167344728530.2425628.9613910866466387722.stgit@warthog.procyon.org.uk/ # v5
> Link: https://lore.kernel.org/r/167391053207.2311931.16398133457201442907.stgit@warthog.procyon.org.uk/ # v6
> ---
>
> Notes:
> ver #7)
> - Switch to passing in iter-specific flags rather than FOLL_* flags.
> - Drop the direction flags for now.
> - Use ITER_ALLOW_P2PDMA to request FOLL_PCI_P2PDMA.
> - Disallow use of ITER_ALLOW_P2PDMA with non-user-backed iter.
> - Add support for extraction from KVEC-type iters.
> - Use iov_iter_advance() rather than open-coding it.
> - Make BVEC- and KVEC-type skip over initial empty vectors.
>
> ver #6)
> - Add back the function to indicate the cleanup mode.
> - Drop the cleanup_mode return arg to iov_iter_extract_pages().
> - Pass FOLL_SOURCE/DEST_BUF in gup_flags. Check this against the iter
> data_source.
>
> ver #4)
> - Use ITER_SOURCE/DEST instead of WRITE/READ.
> - Allow additional FOLL_* flags, such as FOLL_PCI_P2PDMA to be passed in.
>
> ver #3)
> - Switch to using EXPORT_SYMBOL_GPL to prevent indirect 3rd-party access
> to get/pin_user_pages_fast()[1].
>
> include/linux/uio.h | 28 +++
> lib/iov_iter.c | 424 ++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 452 insertions(+)
>
> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index 46d5080314c6..a4233049ab7a 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
> @@ -363,4 +363,32 @@ static inline void iov_iter_ubuf(struct iov_iter *i, unsigned int direction,
> /* Flags for iov_iter_get/extract_pages*() */
> #define ITER_ALLOW_P2PDMA 0x01 /* Allow P2PDMA on the extracted pages */
>
> +ssize_t iov_iter_extract_pages(struct iov_iter *i, struct page ***pages,
> + size_t maxsize, unsigned int maxpages,
> + unsigned int extract_flags, size_t *offset0);
> +
> +/**
> + * iov_iter_extract_mode - Indicate how pages from the iterator will be retained
> + * @iter: The iterator
> + * @extract_flags: How the iterator is to be used
> + *
> + * Examine the iterator and @extract_flags and indicate by returning FOLL_PIN,
> + * FOLL_GET or 0 as to how, if at all, pages extracted from the iterator will
> + * be retained by the extraction function.
> + *
> + * FOLL_GET indicates that the pages will have a reference taken on them that
> + * the caller must put. This can be done for DMA/async DIO write from a page.
> + *
> + * FOLL_PIN indicates that the pages will have a pin placed in them that the
> + * caller must unpin. This is must be done for DMA/async DIO read to a page to
> + * avoid CoW problems in fork.
> + *
> + * 0 indicates that no measures are taken and that it's up to the caller to
> + * retain the pages.
> + */
> +#define iov_iter_extract_mode(iter, extract_flags) \
> + (user_backed_iter(iter) ? \
> + (iter->data_source == ITER_SOURCE) ? \
> + FOLL_GET : FOLL_PIN : 0)
> +
>
How does this work align with the goal of no longer using FOLL_GET for
O_DIRECT? We should get rid of any FOLL_GET usage for accessing page
content.
@John, any comments?
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2023-01-23 11:29 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-20 17:55 [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list) David Howells
2023-01-20 17:55 ` [PATCH v7 1/8] iov_iter: Define flags to qualify page extraction David Howells
2023-01-21 13:01 ` Christoph Hellwig
2023-01-20 17:55 ` [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator David Howells
2023-01-21 13:01 ` Christoph Hellwig
2023-01-21 13:10 ` Christoph Hellwig
2023-01-21 13:30 ` David Howells
2023-01-21 13:33 ` Christoph Hellwig
2023-01-23 11:28 ` David Hildenbrand [this message]
2023-01-23 11:51 ` David Howells
2023-01-23 13:11 ` David Hildenbrand
2023-01-23 13:19 ` David Howells
2023-01-23 13:24 ` David Hildenbrand
2023-01-23 19:56 ` John Hubbard
2023-01-26 22:15 ` Al Viro
2023-01-26 23:41 ` David Hildenbrand
2023-01-27 0:05 ` David Howells
2023-01-27 0:20 ` David Hildenbrand
2023-01-23 13:38 ` David Howells
2023-01-23 14:20 ` David Hildenbrand
2023-01-23 14:48 ` Christoph Hellwig
2023-01-23 16:11 ` Jan Kara
2023-01-23 16:17 ` Christoph Hellwig
2023-01-23 23:07 ` John Hubbard
2023-01-24 5:57 ` Christoph Hellwig
2023-01-24 6:55 ` John Hubbard
2023-01-23 12:00 ` David Howells
2023-01-20 17:55 ` [PATCH v7 3/8] mm: Provide a helper to drop a pin/ref on a page David Howells
2023-01-20 17:55 ` [PATCH v7 4/8] block: Rename BIO_NO_PAGE_REF to BIO_PAGE_REFFED and invert the meaning David Howells
2023-01-21 13:04 ` Christoph Hellwig
2023-01-23 9:38 ` David Howells
2023-01-23 9:56 ` Christoph Hellwig
2023-01-20 17:55 ` [PATCH v7 5/8] block: Add BIO_PAGE_PINNED David Howells
2023-01-21 13:05 ` Christoph Hellwig
2023-01-20 17:55 ` [PATCH v7 6/8] block: Make bio structs pin pages rather than ref'ing if appropriate David Howells
2023-01-21 13:07 ` Christoph Hellwig
2023-01-23 11:28 ` David Howells
2023-01-23 14:49 ` Christoph Hellwig
2023-01-20 17:55 ` [PATCH v7 7/8] block: Fix bio_flagged() so that gcc can better optimise it David Howells
2023-01-20 17:55 ` [PATCH v7 8/8] mm: Renumber FOLL_GET and FOLL_PIN down David Howells
2023-01-20 18:59 ` Matthew Wilcox
2023-01-20 19:18 ` David Howells
2023-01-23 16:31 ` [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list) Matthew Wilcox
2023-01-23 16:42 ` Jan Kara
2023-01-23 17:33 ` Matthew Wilcox
2023-01-23 22:53 ` John Hubbard
2023-01-24 10:29 ` Jan Kara
2023-01-24 13:21 ` Christoph Hellwig
2023-01-23 16:38 ` David Howells
2023-01-23 16:42 ` Matthew Wilcox
2023-01-23 17:25 ` Jan Kara
2023-01-24 10:24 ` David Hildenbrand
2023-01-23 17:19 ` David Howells
2023-01-23 18:04 ` Matthew Wilcox
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