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. -- 2.0.0 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
