linux-kernel - [PATCH v2 0/4] introduce get_user_pages

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Date:   Tue, 14 Nov 2017 11:56:29 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     akpm@...ux-foundation.org
Cc:     Inki Dae <inki.dae@...sung.com>, Jan Kara <jack@...e.cz>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        linux-nvdimm@...ts.01.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Jeff Moyer <jmoyer@...hat.com>, stable@...r.kernel.org,
        Hal Rosenstock <hal.rosenstock@...il.com>,
        Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        linux-mm@...ck.org, Doug Ledford <dledford@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Sean Hefty <sean.hefty@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Vlastimil Babka <vbabka@...e.cz>, linux-media@...r.kernel.org
Subject: [PATCH v2 0/4] introduce get_user_pages_longterm()

Changes since v1 [1]:
* Cleanup local 'vmas' argument (Christoph)
* Replace inline IS_ENABLED(CONFIG_FS_DAX) in C code with ifdef
  versions of get_user_pages_longterm() for the FS_DAX on/off cases
  (Christoph)
* Add a new patch for the get_vaddr_frames() case, this impacts users
  like V4L2, and the Exynos driver.
* Collect Christoph's reviewed-by for the rdma change

[1]: https://lwn.net/Articles/738323/

---

Andrew,

Here is a new get_user_pages api for cases where a driver intends to
keep an elevated page count indefinitely. This is distinct from usages
like iov_iter_get_pages where the elevated page counts are transient.
The iov_iter_get_pages cases immediately turn around and submit the
pages to a device driver which will put_page when the i/o operation
completes (under kernel control).

In the longterm case userspace is responsible for dropping the page
reference at some undefined point in the future. This is untenable for
filesystem-dax case where the filesystem is in control of the lifetime
of the block / page and needs reasonable limits on how long it can wait
for pages in a mapping to become idle.

Fixing filesystems to actually wait for dax pages to be idle before
blocks from a truncate/hole-punch operation are repurposed is saved for
a later patch series.

Also, allowing longterm registration of dax mappings is a future patch
series that introduces a "map with lease" semantic where the kernel can
revoke a lease and force userspace to drop its page references.

I have also tagged these for -stable to purposely break cases that might
assume that longterm memory registrations for filesystem-dax mappings
were supported by the kernel. The behavior regression this policy change
implies is one of the reasons we maintain the "dax enabled. Warning:
EXPERIMENTAL, use at your own risk" notification when mounting a
filesystem in dax mode.

It is worth noting the device-dax interface does not suffer the same
constraints since it does not support file space management operations
like hole-punch.

---

Dan Williams (4):
      mm: introduce get_user_pages_longterm
      mm: fail get_vaddr_frames() for filesystem-dax mappings
      [media] v4l2: disable filesystem-dax mapping support
      IB/core: disable memory registration of fileystem-dax vmas

 drivers/infiniband/core/umem.c            |    2 -
 drivers/media/v4l2-core/videobuf-dma-sg.c |    5 +-
 include/linux/fs.h                        |   14 ++++++
 include/linux/mm.h                        |   13 ++++++
 mm/frame_vector.c                         |    4 ++
 mm/gup.c                                  |   64 +++++++++++++++++++++++++++++
 6 files changed, 99 insertions(+), 3 deletions(-)