[PATCH v7 00/14] dax: fix dma vs truncate/hole-punch

Dan Williams
2018-03-21T23:06:21Z

Changes since v6 [1]:
* Collect some more reviewed-by's from Christoph, thanks Christoph!

* Rework XFS_MMAPLOCK_EXCL handling to *permit* rather than require
XFS_MMAPLOCK_EXCL to be held over calls to xfs_break_layouts()
(Christoph)

* Clean up kbuild robot reports against "ext2, dax: introduce ext2_dax_aops"
and "fs, dax: use page->mapping to warn if truncate collides with a
busy page".

* Squash the xfs_break_leased_layouts() 'did_unlock' rework into "xfs:
prepare xfs_break_layouts() for another layout type" (Christoph)

* Communicate the 'did_unlock' condition with an out parameter rather
than a positive error code (Christoph). A few other small / welcome
clean ups fell out as a result.

* Rename BREAK_TRUNCATE to BREAK_UNMAPI to make it clear the
implementation is concerned with any and all inode extent unmap
events, not just truncate(2). (Darrick)

* Rebase the branch on commit 6b2bb7265f0b "sched/wait: Introduce
wait_var_event()" from tip.git/sched/wait. Thanks Peter!

[1]: https://lists.01.org/pipermail/linux-nvdimm/2018-March/014806.html

---

Background:

get_user_pages() in the filesystem pins file backed memory pages for
access by devices performing dma. However, it only pins the memory pages
not the page-to-file offset association. If a file is truncated the
pages are mapped out of the file and dma may continue indefinitely into
a page that is owned by a device driver. This breaks coherency of the
file vs dma, but the assumption is that if userspace wants the
file-space truncated it does not matter what data is inbound from the
device, it is not relevant anymore. The only expectation is that dma can
safely continue while the filesystem reallocates the block(s).

Problem:

This expectation that dma can safely continue while the filesystem
changes the block map is broken by dax. With dax the target dma page
*is* the filesystem block. The model of leaving the page pinned for dma,
but truncating the file block out of the file, means that the filesytem
is free to reallocate a block under active dma to another file and now
the expected data-incoherency situation has turned into active
data-corruption.

Solution:

Defer all filesystem operations (fallocate(), truncate()) on a dax mode
file while any page/block in the file is under active dma. This solution
assumes that dma is transient. Cases where dma operations are known to
not be transient, like RDMA, have been explicitly disabled via
commits like 5f1d43de5416 "IB/core: disable memory registration of
filesystem-dax vmas".

The dax_layout_busy_page() routine is called by filesystems with a lock
held against mm faults (i_mmap_lock) to find pinned / busy dax pages.
The process of looking up a busy page invalidates all mappings
to trigger any subsequent get_user_pages() to block on i_mmap_lock.
The filesystem continues to call dax_layout_busy_page() until it finally
returns no more active pages. This approach assumes that the page
pinning is transient, if that assumption is violated the system would
have likely hung from the uncompleted I/O.

---

Dan Williams (14):
dax: store pfns in the radix
fs, dax: prepare for dax-specific address_space_operations
block, dax: remove dead code in blkdev_writepages()
xfs, dax: introduce xfs_dax_aops
ext4, dax: introduce ext4_dax_aops
ext2, dax: introduce ext2_dax_aops
fs, dax: use page->mapping to warn if truncate collides with a busy page
mm, dax: enable filesystems to trigger dev_pagemap ->page_free callbacks
mm, dev_pagemap: introduce CONFIG_DEV_PAGEMAP_OPS
memremap: mark devm_memremap_pages() EXPORT_SYMBOL_GPL
mm, fs, dax: handle layout changes to pinned dax mappings
xfs: prepare xfs_break_layouts() to be called with XFS_MMAPLOCK_EXCL
xfs: prepare xfs_break_layouts() for another layout type
xfs, dax: introduce xfs_break_dax_layouts()

drivers/dax/super.c | 96 +++++++++++++++++--
drivers/nvdimm/pmem.c | 3 -
fs/Kconfig | 1
fs/block_dev.c | 5 -
fs/dax.c | 231 ++++++++++++++++++++++++++++++++++++----------
fs/ext2/ext2.h | 1
fs/ext2/inode.c | 43 +++++----
fs/ext2/namei.c | 18 ----
fs/ext2/super.c | 6 +
fs/ext4/inode.c | 38 ++++++--
fs/ext4/super.c | 6 +
fs/libfs.c | 27 +++++
fs/xfs/xfs_aops.c | 21 +++-
fs/xfs/xfs_aops.h | 1
fs/xfs/xfs_file.c | 76 ++++++++++++++-
fs/xfs/xfs_inode.h | 16 +++
fs/xfs/xfs_ioctl.c | 8 --
fs/xfs/xfs_iops.c | 21 +++-
fs/xfs/xfs_pnfs.c | 16 ++-
fs/xfs/xfs_pnfs.h | 6 +
fs/xfs/xfs_super.c | 20 ++--
include/linux/dax.h | 46 ++++++++-
include/linux/fs.h | 3 +
include/linux/memremap.h | 28 ++----
include/linux/mm.h | 61 +++++++++---
kernel/memremap.c | 32 +++++-
mm/Kconfig | 5 +
mm/gup.c | 5 +
mm/hmm.c | 13 ---
mm/swap.c | 3 -
30 files changed, 638 insertions(+), 218 deletions(-)