[PATCH v11 11/21] dax: Replace XIP documentation with DAX documentation

[Matthew Wilcox]

From: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>

Based on the original XIP documentation, this documents the current
state of affairs, and includes instructions on how users can enable DAX
if their devices and kernel support it.

Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
Reviewed-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
---
 Documentation/filesystems/dax.txt | 89 +++++++++++++++++++++++++++++++++++++++
 Documentation/filesystems/xip.txt | 71 -------------------------------
 2 files changed, 89 insertions(+), 71 deletions(-)
 create mode 100644 Documentation/filesystems/dax.txt
 delete mode 100644 Documentation/filesystems/xip.txt

diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
new file mode 100644
index 0000000..635adaa
--- /dev/null
+++ b/Documentation/filesystems/dax.txt
@@ -0,0 +1,89 @@
+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.  When mounting it, use the -o dax option manually
+or add 'dax' to the options in /etc/fstab.
+
+
+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:
+- axonram: Axon DDR2 device driver
+- brd: RAM backed block device driver
+- dcssblk: s390 dcss block device 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 the direct_IO address space operation, and calling
+  dax_do_io() instead of blockdev_direct_IO() if S_DAX is set
+- implementing an mmap file operation for DAX files which sets the
+  VM_MIXEDMAP flag on the VMA, and setting the vm_ops to include handlers
+  for fault and page_mkwrite (which should probably call dax_fault() and
+  dax_mkwrite(), passing the appropriate get_block() callback)
+- calling dax_truncate_page() instead of block_truncate_page() for DAX files
+- ensuring that there is sufficient locking between reads, writes,
+  truncates and page faults
+
+The get_block() callback passed to the DAX functions may return
+uninitialised extents.  If it does, it must ensure that simultaneous
+calls to get_block() (for example by a page-fault racing with a read()
+or a write()) work correctly.
+
+These filesystems may be used for inspiration:
+- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
+
+
+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.
+
+Calling get_user_pages() on a range of user memory that has been mmaped
+from a DAX file will fail as there are no 'struct page' to describe
+those pages.  This problem is being worked on.  That means that 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 include RDMA, sendfile() and splice().
diff --git a/Documentation/filesystems/xip.txt b/Documentation/filesystems/xip.txt
deleted file mode 100644
index b774729..0000000
--- a/Documentation/filesystems/xip.txt
+++ /dev/null
@@ -1,71 +0,0 @@
-Execute-in-place for file mappings
-----------------------------------
-
-Motivation
-----------
-File mappings are performed by mapping page cache pages to userspace. In
-addition, read&write type file operations also transfer data from/to the page
-cache.
-
-For memory backed storage devices that use the block device interface, the page
-cache pages are in fact copies of the original storage. Various approaches
-exist to work around the need for an extra copy. The ramdisk driver for example
-does read the data into the page cache, keeps a reference, and discards the
-original data behind later on.
-
-Execute-in-place solves this issue the other way around: instead of keeping
-data in the page cache, the need to have a page cache copy is eliminated
-completely. With execute-in-place, read&write type operations are performed
-directly from/to the memory backed storage device. For file mappings, the
-storage device itself is mapped directly into userspace.
-
-This implementation was initially written for shared memory segments between
-different virtual machines on s390 hardware to allow multiple machines to
-share the same binaries and libraries.
-
-Implementation
---------------
-Execute-in-place is implemented in three steps: block device operation,
-address space operation, and file operations.
-
-A block device operation named direct_access is used to translate the
-block device sector number 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.
-
-The block device operation is optional, these block devices support it as of
-today:
-- dcssblk: s390 dcss block device driver
-
-An address space operation named get_xip_mem is used to retrieve references
-to a page frame number and a kernel address. To obtain these values a reference
-to an address_space is provided. This function assigns values to the kmem and
-pfn parameters. The third argument indicates whether the function should allocate
-blocks if needed.
-
-This address space operation is mutually exclusive with readpage&writepage that
-do page cache read/write operations.
-The following filesystems support it as of today:
-- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
-
-A set of file operations that do utilize get_xip_page can be found in
-mm/filemap_xip.c . The following file operation implementations are provided:
-- aio_read/aio_write
-- readv/writev
-- sendfile
-
-The generic file operations do_sync_read/do_sync_write can be used to implement
-classic synchronous IO calls.
-
-Shortcomings
-------------
-This implementation is limited to storage devices that are cpu addressable at
-all times (no highmem or such). It works well on rom/ram, but enhancements are
-needed to make it work with flash in read+write mode.
-Putting the Linux kernel and/or its modules on a xip filesystem does not mean
-they are not copied.
-- 
2.1.0

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