Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator

[David Hildenbrand <david@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