[PATCH v6 00/15] dax: fix dma vs truncate/hole-punch

Dan Williams
2018-03-15T16:00:46Z

Changes since v5 [1]:
* Split the introduction of dax-specific address_space_operations into
its own patch, and place them in fs/libfs.c (Christoph)

* Kill some more straggling dead code implementing dax support for block
devices.

* Mark devm_memremap_pages EXPORT_SYMBOL_GPL (Christoph)

* Introduce {xfs,ext4,ext2}_dax_writepages() and kill the dynamic check
for IS_DAX() in the typical _writepages() implementations in these
filesystems. (Christoph)

* Rework xfs_break_layouts() to assume the XFS_MMAPLOCK_EXCL is held at
entry. (Christoph)

* Replace the XFS_BREAK_WRITE and XFS_BREAK_MAPS flags with the
BREAK_WRITE and BREAK_TRUNCATE enum values since BREAK_WRITE is a
subset of BREAK_TRUNCATE. (Christoph)

* Replace wait_for_atomic_one() with Peter's new wait_var_event()
facility [2]. (Peter)

[1]: https://lists.01.org/pipermail/linux-nvdimm/2018-March/014585.html
[2]: https://patchwork.kernel.org/patch/10284383/

---

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 (15):
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: require mmap lock for xfs_break_layouts()
xfs: communicate lock drop events from xfs_break_layouts()
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 | 232 ++++++++++++++++++++++++++++++++++++----------
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 | 87 ++++++++++++++++-
fs/xfs/xfs_inode.h | 16 +++
fs/xfs/xfs_ioctl.c | 8 --
fs/xfs/xfs_iops.c | 21 +++-
fs/xfs/xfs_pnfs.c | 17 ++-
fs/xfs/xfs_pnfs.h | 4 -
fs/xfs/xfs_super.c | 20 ++--
include/linux/dax.h | 51 +++++++++-
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, 653 insertions(+), 219 deletions(-)