[PATCH v4 0/3] block: Make old dio use iov_iter_extract_pages() and page pinning

[David Howells <dhowells@redhat.com>]

Hi Christoph, David, Lorenzo,

Here are three patches that go on top of the similar patches for bio
structs now in the block tree that make the old block direct-IO code use
iov_iter_extract_pages() and page pinning.

There are three patches:

 (1) Make page pinning neither add nor remove a pin to/from a ZERO_PAGE,
     thereby allowing the dio code to insert zero pages in the middle of
     dealing with pinned pages.  This also mitigates a potential problem
     whereby userspace could force the overrun the pin counter of a zero
     page.

     A pair of functions are provided to wrap the testing of a page or
     folio to see if it is a zero page.

 (2) Provide a function to allow an additional pin to be taken on a page we
     already have pinned (and do nothing for a zero page).

 (3) Switch direct-io.c over to using page pinning and to use
     iov_iter_extract_pages() so that pages from non-user-backed iterators
     aren't pinned.

I've pushed the patches here also:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=iov-old-dio

David

Changes
=======
ver #4)
 - Use _inc rather than _add ops when we're just adding 1.

ver #3)
 - Move is_zero_page() and is_zero_folio() to mm.h for dependency reasons.
 - Add more comments and adjust the docs about pinning zero pages.
 - Rename page_get_additional_pin() to folio_add_pin().
 - Use is_zero_folio() in folio_add_pin().
 - Rename need_unpin to is_pinned in struct dio.

ver #2)
 - Fix use of ZERO_PAGE().
 - Add wrappers for testing if a page is a zero page.
 - Return the zero page obtained, not ZERO_PAGE(0) unconditionally.
 - Need to set BIO_PAGE_PINNED conditionally, and not BIO_PAGE_REFFED.

Link: https://lore.kernel.org/r/ZGxfrOLZ4aN9/MvE@infradead.org/ [1]
Link: https://lore.kernel.org/r/20230525155102.87353-1-dhowells@redhat.com/ # v1
Link: https://lore.kernel.org/r/20230525223953.225496-1-dhowells@redhat.com/ # v2
Link: https://lore.kernel.org/r/20230526112859.654506-1-dhowells@redhat.com/ # v3

David Howells (3):
  mm: Don't pin ZERO_PAGE in pin_user_pages()
  mm: Provide a function to get an additional pin on a page
  block: Use iov_iter_extract_pages() and page pinning in direct-io.c

 Documentation/core-api/pin_user_pages.rst |  6 ++
 fs/direct-io.c                            | 72 ++++++++++++++---------
 include/linux/mm.h                        | 27 ++++++++-
 mm/gup.c                                  | 58 +++++++++++++++++-
 4 files changed, 131 insertions(+), 32 deletions(-)