Subject: [PATCH v6 00/24] mm/gup: track dma-pinned pages: FOLL_PIN

From: John Hubbard <[email-masked]>
Date: Tue, 19 Nov 2019 00:16:19 -0800

Hi,

Christoph Hellwig has a preference to do things a little differently,
for the devmap cleanup in patch 5 ("mm: devmap: refactor 1-based
refcounting for ZONE_DEVICE pages"). That came up in a different
review thread, because the patch is out for review in two locations.
Here's that review thread:

    https://lore.kernel.org/r/[email-masked]

...and I'm hoping that we can defer that request, because otherwise
it derails this series, which is starting to otherwise look like
it could be ready for 5.5.

There is a git repo and branch, for convenience:

    [email-masked]:johnhubbard/linux.git pin_user_pages_tracking_v6

Changes since v5:

* Fixed the refcounting for huge pages: in most cases, it was
  only taking one GUP_PIN_COUNTING_BIAS's worth of refs, when it
  should have been taking one GUP_PIN_COUNTING_BIAS for each subpage.

  (Much thanks to Jan Kara for spotting that one!)

* Renamed user_page_ref_inc() to try_pin_page(), and added a new
  try_pin_compound_head(). This definitely improves readability.

* Factored out some more duplication in the FOLL_PIN and FOLL_GET
  cases, in gup.c.

* Fixed up some straggling "get_" --> "pin_" references in the comments.

* Added reviewed-by tags.

Changes since v4:

* Renamed put_user_page*() --> unpin_user_page().

* Removed all pin_longterm_pages*() calls. We will use FOLL_LONGTERM
  at the call sites. (FOLL_PIN, however, remains an internal gup flag).

  This is very nice: many patches just change three characters now:
  get_user_pages --> pin_user_pages. I think we've found the right
  balance of wrapper calls and gup flags, for the call sites.

* Updated a lot of documentation and commit logs to match the above
  two large changes.

* Changed gup_benchmark tests and run_vmtests, to adapt to one less
  use case: there is no pin_longterm_pages() call anymore.

* This includes a new devmap cleanup patch from Dan Williams, along
  with a rebased follow-up: patches 4 and 5, already mentioned above.

* Fixed patch 10 ("mm/gup: introduce pin_user_pages*() and FOLL_PIN"),
  so as to make pin_user_pages*() calls act as placeholders for the
  corresponding get_user_pages*() calls, until a later patch fully
  implements the DMA-pinning functionality.

  Thanks to Jan Kara for noticing that.

* Fixed the implementation of pin_user_pages_remote().

* Further tweaked patch 2 ("mm/gup: factor out duplicate code from four
  routines"), in response to Jan Kara's feedback.

* Dropped a few reviewed-by tags  due to changes that invalidated
  them.

Changes since v3:

* VFIO fix (patch 8): applied further cleanup: removed a pre-existing,
  unnecessary release and reacquire of mmap_sem. Moved the DAX vma
  checks from the vfio call site, to gup internals, and added comments
  (and commit log) to clarify.

* Due to the above, made a corresponding fix to the
  pin_longterm_pages_remote(), which was actually calling the wrong
  gup internal function.

* Changed put_user_page() comments, to refer to pin*() APIs, rather than
  get_user_pages*() APIs.

* Reverted an accidental whitespace-only change in the IB ODP code.

* Added a few more reviewed-by tags.

Changes since v2:

* Added a patch to convert IB/umem from normal gup, to gup_fast(). This
  is also posted separately, in order to hopefully get some runtime
  testing.

* Changed the page devmap code to be a little clearer,
  thanks to Jerome for that.

* Split out the page devmap changes into a separate patch (and moved
  Ira's Signed-off-by to that patch).

* Fixed my bug in IB: ODP code does not require pin_user_pages()
  semantics. Therefore, revert the put_user_page() calls to put_page(),
  and leave the get_user_pages() call as-is.

      * As part of the revert, I am proposing here a change directly
        from put_user_pages(), to release_pages(). I'd feel better if
        someone agrees that this is the best way. It uses the more
        efficient release_pages(), instead of put_page() in a loop,
        and keep the change to just a few character on one line,
        but OTOH it is not a pure revert.

* Loosened the FOLL_LONGTERM restrictions in the
  __get_user_pages_locked() implementation, and used that in order
  to fix up a VFIO bug. Thanks to Jason for that idea.

    * Note the use of release_pages() in IB: is that OK?

* Added a few more WARN's and clarifying comments nearby.

* Many documentation improvements in various comments.

* Moved the new pin_user_pages.rst from Documentation/vm/ to
  Documentation/core-api/ .

* Commit descriptions: added clarifying notes to the three patches
  (drm/via, fs/io_uring, net/xdp) that already had put_user_page()
  calls in place.

* Collected all pending Reviewed-by and Acked-by tags, from v1 and v2
  email threads.

* Lot of churn from v2 --> v3, so it's possible that new bugs
  sneaked in.

NOT DONE: separate patchset is required:

* __get_user_pages_locked(): stop compensating for
  buggy callers who failed to set FOLL_GET. Instead, assert
  that FOLL_GET is set (and fail if it's not).

======================================================================
Original cover letter (edited to fix up the patch description numbers)

This applies cleanly to linux-next and mmotm, and also to linux.git if
linux-next's commit 20cac10710c9 ("mm/gup_benchmark: fix MAP_HUGETLB
case") is first applied there.

This provides tracking of dma-pinned pages. This is a prerequisite to
solving the larger problem of proper interactions between file-backed
pages, and [R]DMA activities, as discussed in [1], [2], [3], and in
a remarkable number of email threads since about 2017. :)

A new internal gup flag, FOLL_PIN is introduced, and thoroughly
documented in the last patch's Documentation/vm/pin_user_pages.rst.

I believe that this will provide a good starting point for doing the
layout lease work that Ira Weiny has been working on. That's because
these new wrapper functions provide a clean, constrained, systematically
named set of functionality that, again, is required in order to even
know if a page is "dma-pinned".

In contrast to earlier approaches, the page tracking can be
incrementally applied to the kernel call sites that, until now, have
been simply calling get_user_pages() ("gup"). In other words, opt-in by
changing from this:

    get_user_pages() (sets FOLL_GET)
    put_page()

to this:
    pin_user_pages() (sets FOLL_PIN)
    put_user_page()

Because there are interdependencies with FOLL_LONGTERM, a similar
conversion as for FOLL_PIN, was applied. The change was from this:

    get_user_pages(FOLL_LONGTERM) (also sets FOLL_GET)
    put_page()

to this:
    pin_longterm_pages() (sets FOLL_PIN | FOLL_LONGTERM)
    put_user_page()

============================================================
Patch summary:

* Patches 1-9: refactoring and preparatory cleanup, independent fixes

* Patch 10: introduce pin_user_pages(), FOLL_PIN, but no functional
           changes yet
* Patches 11-16: Convert existing put_user_page() callers, to use the
                 new pin*()
* Patch 17: Activate tracking of FOLL_PIN pages.
* Patches 18-20: convert various callers
* Patches: 21-23: gup_benchmark and run_vmtests support
* Patch 24: rename put_user_page*() --> unpin_user_page*()

============================================================
Testing:

* I've done some overall kernel testing (LTP, and a few other goodies),
  and some directed testing to exercise some of the changes. And as you
  can see, gup_benchmark is enhanced to exercise this. Basically, I've been
  able to runtime test the core get_user_pages() and pin_user_pages() and
  related routines, but not so much on several of the call sites--but those
  are generally just a couple of lines changed, each.

  Not much of the kernel is actually using this, which on one hand
  reduces risk quite a lot. But on the other hand, testing coverage
  is low. So I'd love it if, in particular, the Infiniband and PowerPC
  folks could do a smoke test of this series for me.

  Also, my runtime testing for the call sites so far is very weak:

    * io_uring: Some directed tests from liburing exercise this, and they pass.
    * process_vm_access.c: A small directed test passes.
    * gup_benchmark: the enhanced version hits the new gup.c code, and passes.
    * infiniband (still only have crude "IB pingpong" working, on a
                  good day: it's not exercising my conversions at runtime...)
    * VFIO: compiles (I'm vowing to set up a run time test soon, but it's
                      not ready just yet)
    * powerpc: it compiles...
    * drm/via: compiles...
    * goldfish: compiles...
    * net/xdp: compiles...
    * media/v4l2: compiles...

============================================================
Next:

* Get the block/bio_vec sites converted to use pin_user_pages().

* Work with Ira and Dave Chinner to weave this together with the
  layout lease stuff.

============================================================

[1] Some slow progress on get_user_pages() (Apr 2, 2019): https://lwn.net/Articles/784574/
[2] DMA and get_user_pages() (LPC: Dec 12, 2018): https://lwn.net/Articles/774411/
[3] The trouble with get_user_pages() (Apr 30, 2018): https://lwn.net/Articles/753027/

Dan Williams (1):
  mm: Cleanup __put_devmap_managed_page() vs ->page_free()

John Hubbard (23):
  mm/gup: pass flags arg to __gup_device_* functions
  mm/gup: factor out duplicate code from four routines
  mm/gup: move try_get_compound_head() to top, fix minor issues
  mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages
  goldish_pipe: rename local pin_user_pages() routine
  IB/umem: use get_user_pages_fast() to pin DMA pages
  media/v4l2-core: set pages dirty upon releasing DMA buffers
  vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM
  mm/gup: introduce pin_user_pages*() and FOLL_PIN
  goldish_pipe: convert to pin_user_pages() and put_user_page()
  IB/{core,hw,umem}: set FOLL_PIN via pin_user_pages*(), fix up ODP
  mm/process_vm_access: set FOLL_PIN via pin_user_pages_remote()
  drm/via: set FOLL_PIN via pin_user_pages_fast()
  fs/io_uring: set FOLL_PIN via pin_user_pages()
  net/xdp: set FOLL_PIN via pin_user_pages()
  mm/gup: track FOLL_PIN pages
  media/v4l2-core: pin_user_pages (FOLL_PIN) and put_user_page()
    conversion
  vfio, mm: pin_user_pages (FOLL_PIN) and put_user_page() conversion
  powerpc: book3s64: convert to pin_user_pages() and put_user_page()
  mm/gup_benchmark: use proper FOLL_WRITE flags instead of hard-coding
    "1"
  mm/gup_benchmark: support pin_user_pages() and related calls
  selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN
    coverage
  mm, tree-wide: rename put_user_page*() to unpin_user_page*()

 Documentation/core-api/index.rst            |   1 +
 Documentation/core-api/pin_user_pages.rst   | 233 +++++++++
 arch/powerpc/mm/book3s64/iommu_api.c        |  12 +-
 drivers/gpu/drm/via/via_dmablit.c           |   6 +-
 drivers/infiniband/core/umem.c              |  19 +-
 drivers/infiniband/core/umem_odp.c          |  13 +-
 drivers/infiniband/hw/hfi1/user_pages.c     |   4 +-
 drivers/infiniband/hw/mthca/mthca_memfree.c |   8 +-
 drivers/infiniband/hw/qib/qib_user_pages.c  |   4 +-
 drivers/infiniband/hw/qib/qib_user_sdma.c   |   8 +-
 drivers/infiniband/hw/usnic/usnic_uiom.c    |   4 +-
 drivers/infiniband/sw/siw/siw_mem.c         |   4 +-
 drivers/media/v4l2-core/videobuf-dma-sg.c   |   8 +-
 drivers/nvdimm/pmem.c                       |   6 -
 drivers/platform/goldfish/goldfish_pipe.c   |  35 +-
 drivers/vfio/vfio_iommu_type1.c             |  35 +-
 fs/io_uring.c                               |   6 +-
 include/linux/mm.h                          | 168 +++++-
 include/linux/mmzone.h                      |   2 +
 include/linux/page_ref.h                    |  10 +
 mm/gup.c                                    | 548 +++++++++++++++-----
 mm/gup_benchmark.c                          |  74 ++-
 mm/huge_memory.c                            |  54 +-
 mm/hugetlb.c                                |  39 +-
 mm/memremap.c                               |  76 ++-
 mm/process_vm_access.c                      |  28 +-
 mm/vmstat.c                                 |   2 +
 net/xdp/xdp_umem.c                          |   4 +-
 tools/testing/selftests/vm/gup_benchmark.c  |  21 +-
 tools/testing/selftests/vm/run_vmtests      |  22 +
 30 files changed, 1104 insertions(+), 350 deletions(-)
 create mode 100644 Documentation/core-api/pin_user_pages.rst

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2.24.0