258 lines
9.8 KiB
Plaintext
258 lines
9.8 KiB
Plaintext
Direct Access for files
|
|
-----------------------
|
|
|
|
Motivation
|
|
----------
|
|
|
|
The page cache is usually used to buffer reads and writes to files.
|
|
It is also used to provide the pages which are mapped into userspace
|
|
by a call to mmap.
|
|
|
|
For block devices that are memory-like, the page cache pages would be
|
|
unnecessary copies of the original storage. The DAX code removes the
|
|
extra copy by performing reads and writes directly to the storage device.
|
|
For file mappings, the storage device is mapped directly into userspace.
|
|
|
|
|
|
Usage
|
|
-----
|
|
|
|
If you have a block device which supports DAX, you can make a filesystem
|
|
on it as usual. The DAX code currently only supports files with a block
|
|
size equal to your kernel's PAGE_SIZE, so you may need to specify a block
|
|
size when creating the filesystem.
|
|
|
|
Currently 3 filesystems support DAX: ext2, ext4 and xfs. Enabling DAX on them
|
|
is different.
|
|
|
|
Enabling DAX on ext2
|
|
-----------------------------
|
|
|
|
When mounting the filesystem, use the "-o dax" option on the command line or
|
|
add 'dax' to the options in /etc/fstab. This works to enable DAX on all files
|
|
within the filesystem. It is equivalent to the '-o dax=always' behavior below.
|
|
|
|
|
|
Enabling DAX on xfs and ext4
|
|
----------------------------
|
|
|
|
Summary
|
|
-------
|
|
|
|
1. There exists an in-kernel file access mode flag S_DAX that corresponds to
|
|
the statx flag STATX_ATTR_DAX. See the manpage for statx(2) for details
|
|
about this access mode.
|
|
|
|
2. There exists a persistent flag FS_XFLAG_DAX that can be applied to regular
|
|
files and directories. This advisory flag can be set or cleared at any
|
|
time, but doing so does not immediately affect the S_DAX state.
|
|
|
|
3. If the persistent FS_XFLAG_DAX flag is set on a directory, this flag will
|
|
be inherited by all regular files and subdirectories that are subsequently
|
|
created in this directory. Files and subdirectories that exist at the time
|
|
this flag is set or cleared on the parent directory are not modified by
|
|
this modification of the parent directory.
|
|
|
|
4. There exist dax mount options which can override FS_XFLAG_DAX in the
|
|
setting of the S_DAX flag. Given underlying storage which supports DAX the
|
|
following hold:
|
|
|
|
"-o dax=inode" means "follow FS_XFLAG_DAX" and is the default.
|
|
|
|
"-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX."
|
|
|
|
"-o dax=always" means "always set S_DAX ignore FS_XFLAG_DAX."
|
|
|
|
"-o dax" is a legacy option which is an alias for "dax=always".
|
|
This may be removed in the future so "-o dax=always" is
|
|
the preferred method for specifying this behavior.
|
|
|
|
NOTE: Modifications to and the inheritance behavior of FS_XFLAG_DAX remain
|
|
the same even when the filesystem is mounted with a dax option. However,
|
|
in-core inode state (S_DAX) will be overridden until the filesystem is
|
|
remounted with dax=inode and the inode is evicted from kernel memory.
|
|
|
|
5. The S_DAX policy can be changed via:
|
|
|
|
a) Setting the parent directory FS_XFLAG_DAX as needed before files are
|
|
created
|
|
|
|
b) Setting the appropriate dax="foo" mount option
|
|
|
|
c) Changing the FS_XFLAG_DAX flag on existing regular files and
|
|
directories. This has runtime constraints and limitations that are
|
|
described in 6) below.
|
|
|
|
6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX
|
|
flag, the change to existing regular files won't take effect until the
|
|
files are closed by all processes.
|
|
|
|
|
|
Details
|
|
-------
|
|
|
|
There are 2 per-file dax flags. One is a persistent inode setting (FS_XFLAG_DAX)
|
|
and the other is a volatile flag indicating the active state of the feature
|
|
(S_DAX).
|
|
|
|
FS_XFLAG_DAX is preserved within the filesystem. This persistent config
|
|
setting can be set, cleared and/or queried using the FS_IOC_FS[GS]ETXATTR ioctl
|
|
(see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'.
|
|
|
|
New files and directories automatically inherit FS_XFLAG_DAX from
|
|
their parent directory _when_ _created_. Therefore, setting FS_XFLAG_DAX at
|
|
directory creation time can be used to set a default behavior for an entire
|
|
sub-tree.
|
|
|
|
To clarify inheritance, here are 3 examples:
|
|
|
|
Example A:
|
|
|
|
mkdir -p a/b/c
|
|
xfs_io -c 'chattr +x' a
|
|
mkdir a/b/c/d
|
|
mkdir a/e
|
|
|
|
dax: a,e
|
|
no dax: b,c,d
|
|
|
|
Example B:
|
|
|
|
mkdir a
|
|
xfs_io -c 'chattr +x' a
|
|
mkdir -p a/b/c/d
|
|
|
|
dax: a,b,c,d
|
|
no dax:
|
|
|
|
Example C:
|
|
|
|
mkdir -p a/b/c
|
|
xfs_io -c 'chattr +x' c
|
|
mkdir a/b/c/d
|
|
|
|
dax: c,d
|
|
no dax: a,b
|
|
|
|
|
|
The current enabled state (S_DAX) is set when a file inode is instantiated in
|
|
memory by the kernel. It is set based on the underlying media support, the
|
|
value of FS_XFLAG_DAX and the filesystem's dax mount option.
|
|
|
|
statx can be used to query S_DAX. NOTE that only regular files will ever have
|
|
S_DAX set and therefore statx will never indicate that S_DAX is set on
|
|
directories.
|
|
|
|
Setting the FS_XFLAG_DAX flag (specifically or through inheritance) occurs even
|
|
if the underlying media does not support dax and/or the filesystem is
|
|
overridden with a mount option.
|
|
|
|
|
|
|
|
Implementation Tips for Block Driver Writers
|
|
--------------------------------------------
|
|
|
|
To support DAX in your block driver, implement the 'direct_access'
|
|
block device operation. It is used to translate the sector number
|
|
(expressed in units of 512-byte sectors) to a page frame number (pfn)
|
|
that identifies the physical page for the memory. It also returns a
|
|
kernel virtual address that can be used to access the memory.
|
|
|
|
The direct_access method takes a 'size' parameter that indicates the
|
|
number of bytes being requested. The function should return the number
|
|
of bytes that can be contiguously accessed at that offset. It may also
|
|
return a negative errno if an error occurs.
|
|
|
|
In order to support this method, the storage must be byte-accessible by
|
|
the CPU at all times. If your device uses paging techniques to expose
|
|
a large amount of memory through a smaller window, then you cannot
|
|
implement direct_access. Equally, if your device can occasionally
|
|
stall the CPU for an extended period, you should also not attempt to
|
|
implement direct_access.
|
|
|
|
These block devices may be used for inspiration:
|
|
- brd: RAM backed block device driver
|
|
- dcssblk: s390 dcss block device driver
|
|
- pmem: NVDIMM persistent memory driver
|
|
|
|
|
|
Implementation Tips for Filesystem Writers
|
|
------------------------------------------
|
|
|
|
Filesystem support consists of
|
|
- adding support to mark inodes as being DAX by setting the S_DAX flag in
|
|
i_flags
|
|
- implementing ->read_iter and ->write_iter operations which use dax_iomap_rw()
|
|
when inode has S_DAX flag set
|
|
- implementing an mmap file operation for DAX files which sets the
|
|
VM_MIXEDMAP and VM_HUGEPAGE flags on the VMA, and setting the vm_ops to
|
|
include handlers for fault, pmd_fault, page_mkwrite, pfn_mkwrite. These
|
|
handlers should probably call dax_iomap_fault() passing the appropriate
|
|
fault size and iomap operations.
|
|
- calling iomap_zero_range() passing appropriate iomap operations instead of
|
|
block_truncate_page() for DAX files
|
|
- ensuring that there is sufficient locking between reads, writes,
|
|
truncates and page faults
|
|
|
|
The iomap handlers for allocating blocks must make sure that allocated blocks
|
|
are zeroed out and converted to written extents before being returned to avoid
|
|
exposure of uninitialized data through mmap.
|
|
|
|
These filesystems may be used for inspiration:
|
|
- ext2: see Documentation/filesystems/ext2.rst
|
|
- ext4: see Documentation/filesystems/ext4/
|
|
- xfs: see Documentation/admin-guide/xfs.rst
|
|
|
|
|
|
Handling Media Errors
|
|
---------------------
|
|
|
|
The libnvdimm subsystem stores a record of known media error locations for
|
|
each pmem block device (in gendisk->badblocks). If we fault at such location,
|
|
or one with a latent error not yet discovered, the application can expect
|
|
to receive a SIGBUS. Libnvdimm also allows clearing of these errors by simply
|
|
writing the affected sectors (through the pmem driver, and if the underlying
|
|
NVDIMM supports the clear_poison DSM defined by ACPI).
|
|
|
|
Since DAX IO normally doesn't go through the driver/bio path, applications or
|
|
sysadmins have an option to restore the lost data from a prior backup/inbuilt
|
|
redundancy in the following ways:
|
|
|
|
1. Delete the affected file, and restore from a backup (sysadmin route):
|
|
This will free the filesystem blocks that were being used by the file,
|
|
and the next time they're allocated, they will be zeroed first, which
|
|
happens through the driver, and will clear bad sectors.
|
|
|
|
2. Truncate or hole-punch the part of the file that has a bad-block (at least
|
|
an entire aligned sector has to be hole-punched, but not necessarily an
|
|
entire filesystem block).
|
|
|
|
These are the two basic paths that allow DAX filesystems to continue operating
|
|
in the presence of media errors. More robust error recovery mechanisms can be
|
|
built on top of this in the future, for example, involving redundancy/mirroring
|
|
provided at the block layer through DM, or additionally, at the filesystem
|
|
level. These would have to rely on the above two tenets, that error clearing
|
|
can happen either by sending an IO through the driver, or zeroing (also through
|
|
the driver).
|
|
|
|
|
|
Shortcomings
|
|
------------
|
|
|
|
Even if the kernel or its modules are stored on a filesystem that supports
|
|
DAX on a block device that supports DAX, they will still be copied into RAM.
|
|
|
|
The DAX code does not work correctly on architectures which have virtually
|
|
mapped caches such as ARM, MIPS and SPARC.
|
|
|
|
Calling get_user_pages() on a range of user memory that has been mmaped
|
|
from a DAX file will fail when there are no 'struct page' to describe
|
|
those pages. This problem has been addressed in some device drivers
|
|
by adding optional struct page support for pages under the control of
|
|
the driver (see CONFIG_NVDIMM_PFN in drivers/nvdimm for an example of
|
|
how to do this). In the non struct page cases O_DIRECT reads/writes to
|
|
those memory ranges from a non-DAX file will fail (note that O_DIRECT
|
|
reads/writes _of a DAX file_ do work, it is the memory that is being
|
|
accessed that is key here). Other things that will not work in the
|
|
non struct page case include RDMA, sendfile() and splice().
|