Re: [PATCH v8 02/10] iov_iter: Add a function to extract a page list from an iterator

[David Hildenbrand <david@redhat.com>]

On 23.01.23 18:29, 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 pin added or nothing,
> depending on the type of iterator.
> 
> Add a second function, iov_iter_extract_mode(), to determine how the
> cleanup should be done.
> 
> There are two cases:
> 
>   (1) ITER_IOVEC or ITER_UBUF iterator.
> 
>       Extracted pages will have pins (FOLL_PIN) obtained on them so that a
>       concurrent fork() will forcibly copy the page so that DMA is done
>       to/from the parent's buffer and is unavailable to/unaffected by the
>       child process.
> 
>       iov_iter_extract_mode() will return FOLL_PIN for this case.  The
>       caller should use something like folio_put_unpin() to dispose of the
>       page.
> 
>   (2) 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.
> 
>       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: David Hildenbrand <david@redhat.com>
> cc: Matthew Wilcox <willy@infradead.org>
> cc: linux-fsdevel@vger.kernel.org
> cc: linux-mm@kvack.org
> ---
> 
> Notes:
>      ver #8)
>       - It seems that all DIO is supposed to be done under FOLL_PIN now, and not
>         FOLL_GET, so switch to only using pin_user_pages() for user-backed
>         iters.
>       - Wrap an argument in brackets in the iov_iter_extract_mode() macro.
>       - Drop the extract_flags argument to iov_iter_extract_mode() for now
>         [hch].
>      
>      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 |  22 +++
>   lib/iov_iter.c      | 320 ++++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 342 insertions(+)
> 
> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index 46d5080314c6..a8165335f8da 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
> @@ -363,4 +363,26 @@ 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
> + *
> + * Examine the iterator and indicate by returning FOLL_PIN or 0 as to how, if
> + * at all, pages extracted from the iterator will be retained by the extraction
> + * function.
> + *
> + * 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 to force fork()
> + * to forcibly copy a page for the child (the parent must retain the original
> + * page).
> + *
> + * 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) (user_backed_iter(iter) ? FOLL_PIN : 0)
> +

Does it make sense to move that to the patch where it is needed? (do we 
need it at all anymore?)

-- 
Thanks,

David / dhildenb