[RFC PATCH v2 0/3] block: Make old dio use iov_iter_extract_pages() and page pinning

[David Howells]

Hi Christoph, David,

Since Christoph asked nicely[1] ;-), 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 not add or 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.

     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 #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@xxxxxxxxxxxxx/ [1]
Link: https://lore.kernel.org/r/20230525155102.87353-1-dhowells@xxxxxxxxxx/ # v1

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

 fs/direct-io.c          | 72 ++++++++++++++++++++++++-----------------
 include/linux/mm.h      |  1 +
 include/linux/pgtable.h | 10 ++++++
 mm/gup.c                | 54 ++++++++++++++++++++++++++++++-
 4 files changed, 107 insertions(+), 30 deletions(-)