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

From: David Howells
Date: Fri May 26 2023 - 17:42:41 EST

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

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:



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@xxxxxxxxxxxxx/ [1]
Link: https://lore.kernel.org/r/20230525155102.87353-1-dhowells@xxxxxxxxxx/ # v1
Link: https://lore.kernel.org/r/20230525223953.225496-1-dhowells@xxxxxxxxxx/ # v2
Link: https://lore.kernel.org/r/20230526112859.654506-1-dhowells@xxxxxxxxxx/ # 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(-)