[PATCH v3 0/3] bio: Direct IO: convert to pin_user_pages_fast()

[John Hubbard]

bio: convert get_user_pages_fast() --> pin_user_pages_fast()

Change generic block/bio Direct IO routines, to acquire FOLL_PIN user
pages via the recently added routines:

    iov_iter_pin_pages()
    iov_iter_pin_pages_alloc()
    pin_page()

This effectively converts several file systems (ext4, for example) that
use the common Direct IO routines.

Change the corresponding page release calls from put_page() to
unpin_user_page().

Change bio_release_pages() to handle FOLL_PIN pages. In fact, after this
patch, that is the *only* type of pages that bio_release_pages()
handles.

Design notes
============

Quite a few approaches have been considered over the years. This one is
inspired by Christoph Hellwig's July, 2019 observation that there are
only 5 ITER_ types, and we can simplify handling of them for Direct IO
[1]. Accordingly, this patch implements the following pseudocode:

Direct IO behavior:

    ITER_IOVEC:
        pin_user_pages_fast();
        break;

    ITER_PIPE:
        for each page:
             pin_page();
        break;

    ITER_KVEC:    // already elevated page refcount, leave alone
    ITER_BVEC:    // already elevated page refcount, leave alone
    ITER_DISCARD: // discard
        return -EFAULT or -ENVALID;

...which works for callers that already have sorted out which case they
are in. Such as, Direct IO in the block/bio layers.

Note that this does leave ITER_KVEC and ITER_BVEC unconverted, for now.

Page acquisition: The iov_iter_get_pages*() routines above are at just
the right level in the call stack: the callers already know which system
to use, and so it's a small change to just drop in the replacement
routines. And it's a fan-in/fan-out point: block/bio call sites for
Direct IO funnel their page acquisitions through the
iov_iter_get_pages*() routines, and there are many other callers of
those. And we can't convert all of the callers at once--too many
subsystems are involved, and it would be a too large and too risky
patch.

Page release: there are already separate release routines: put_page()
vs. unpin_user_page(), so it's already done there.

[1] https://lore.kernel.org/kvm/20190724061750.GA19397@xxxxxxxxxxxxx/

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/



John Hubbard (3):
  mm/gup: introduce pin_page()
  iov_iter: introduce iov_iter_pin_pages*() routines
  bio: convert get_user_pages_fast() --> pin_user_pages_fast()

 block/bio.c          |  24 ++++-----
 block/blk-map.c      |   6 +--
 fs/direct-io.c       |  28 +++++------
 fs/iomap/direct-io.c |   2 +-
 include/linux/mm.h   |   2 +
 include/linux/uio.h  |   5 ++
 lib/iov_iter.c       | 113 ++++++++++++++++++++++++++++++++++++++++---
 mm/gup.c             |  33 +++++++++++++
 8 files changed, 175 insertions(+), 38 deletions(-)

-- 
2.28.0