[PATCH RFC 00/10] RDMA/FS DAX truncate proposal

ira.weiny
2019-06-06T01:45:09Z

From: Ira Weiny <ira.weiny@intel.com>

... V1,000,000 ;-)

Pre-requisites:
John Hubbard's put_user_pages() patch series.[1]
Jan Kara's ext4_break_layouts() fixes[2]

Based on the feedback from LSFmm and the LWN article which resulted. I've
decided to take a slightly different tack on this problem.

The real issue is that there is no use case for a user to have RDMA pinn'ed
memory which is then truncated. So really any solution we present which:

A) Prevents file system corruption or data leaks
...and...
B) Informs the user that they did something wrong

Should be an acceptable solution.

Because this is slightly new behavior. And because this is gonig to be
specific to DAX (because of the lack of a page cache) we have made the user
"opt in" to this behavior.

The following patches implement the following solution.

1) The user has to opt in to allowing GUP pins on a file with a layout lease
(now made visible).
2) GUP will fail (EPERM) if a layout lease is not taken
3) Any truncate or hole punch operation on a GUP'ed DAX page will fail.
4) The user has the option of holding the layout lease to receive a SIGIO for
notification to the original thread that another thread has tried to delete
their data. Furthermore this indicates that if the user needs to GUP the
file again they will need to retake the Layout lease before doing so.

NOTE: If the user releases the layout lease or if it has been broken by another
operation further GUP operations on the file will fail without re-taking the
lease. This means that if a user would like to register pieces of a file and
continue to register other pieces later they would be advised to keep the
layout lease, get a SIGIO notification, and retake the lease.

NOTE2: Truncation of pages which are not actively pinned will succeed. Similar
to accessing an mmap to this area GUP pins of that memory may fail.

A general overview follows for background.

It should be noted that one solution for this problem is to use RDMA's On
Demand Paging (ODP). There are 2 big reasons this may not work.

1) The hardware being used for RDMA may not support ODP
2) ODP may be detrimental to the over all network (cluster or cloud)
performance

Therefore, in order to support RDMA to File system pages without On Demand
Paging (ODP) a number of things need to be done.

1) GUP "longterm" users need to inform the other subsystems that they have
taken a pin on a page which may remain pinned for a very "long time".[3]

2) Any page which is "controlled" by a file system needs to have special
handling. The details of the handling depends on if the page is page cache
fronted or not.

2a) A page cache fronted page which has been pinned by GUP long term can use a
bounce buffer to allow the file system to write back snap shots of the page.
This is handled by the FS recognizing the GUP long term pin and making a copy
of the page to be written back.
NOTE: this patch set does not address this path.

2b) A FS "controlled" page which is not page cache fronted is either easier
to deal with or harder depending on the operation the filesystem is trying
to do.

2ba) [Hard case] If the FS operation _is_ a truncate or hole punch the
FS can no longer use the pages in question until the pin has been
removed. This patch set presents a solution to this by introducing
some reasonable restrictions on user space applications.

2bb) [Easy case] If the FS operation is _not_ a truncate or hole punch
then there is nothing which need be done. Data is Read or Written
directly to the page. This is an easy case which would currently work
if not for GUP long term pins being disabled. Therefore this patch set
need not change access to the file data but does allow for GUP pins
after 2ba above is dealt with.

This patch series and presents a solution for problem 2ba)

[1] https://github.com/johnhubbard/linux/tree/gup_dma_core

[2] ext4/dev branch:

- https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/log/?h=dev

Specific patches:

[2a] ext4: wait for outstanding dio during truncate in nojournal mode

- https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?h=dev&id=82a25b027ca48d7ef197295846b352345853dfa8

[2b] ext4: do not delete unlinked inode from orphan list on failed truncate

- https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?h=dev&id=ee0ed02ca93ef1ecf8963ad96638795d55af2c14

[2c] ext4: gracefully handle ext4_break_layouts() failure during truncate

- https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?h=dev&id=b9c1c26739ec2d4b4fb70207a0a9ad6747e43f4c

[3] The definition of long time is debatable but it has been established
that RDMAs use of pages, minutes or hours after the pin is the extreme case
which makes this problem most severe.

Ira Weiny (10):
fs/locks: Add trace_leases_conflict
fs/locks: Export F_LAYOUT lease to user space
mm/gup: Pass flags down to __gup_device_huge* calls
mm/gup: Ensure F_LAYOUT lease is held prior to GUP'ing pages
fs/ext4: Teach ext4 to break layout leases
fs/ext4: Teach dax_layout_busy_page() to operate on a sub-range
fs/ext4: Fail truncate if pages are GUP pinned
fs/xfs: Teach xfs to use new dax_layout_busy_page()
fs/xfs: Fail truncate if pages are GUP pinned
mm/gup: Remove FOLL_LONGTERM DAX exclusion

fs/Kconfig | 1 +
fs/dax.c | 38 ++++++---
fs/ext4/ext4.h | 2 +-
fs/ext4/extents.c | 6 +-
fs/ext4/inode.c | 26 +++++--
fs/locks.c | 97 ++++++++++++++++++++---
fs/xfs/xfs_file.c | 24 ++++--
fs/xfs/xfs_inode.h | 5 +-
fs/xfs/xfs_ioctl.c | 15 +++-
fs/xfs/xfs_iops.c | 14 +++-
fs/xfs/xfs_pnfs.c | 14 ++--
include/linux/dax.h | 9 ++-
include/linux/fs.h | 2 +-
include/linux/mm.h | 2 +
include/trace/events/filelock.h | 35 +++++++++
include/uapi/asm-generic/fcntl.h | 3 +
mm/gup.c | 129 ++++++++++++-------------------
mm/huge_memory.c | 12 +++
18 files changed, 299 insertions(+), 135 deletions(-)

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2.20.1