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This is the support code for DAX-enabled filesystems to allow them to provide huge pages in response to faults. Signed-off-by: Matthew Wilcox <willy@linux.intel.com> Cc: Hillf Danton <dhillf@gmail.com> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> Cc: Theodore Ts'o <tytso@mit.edu> Cc: Jan Kara <jack@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
98 lines
4.1 KiB
Plaintext
98 lines
4.1 KiB
Plaintext
Direct Access for files
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-----------------------
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Motivation
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----------
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The page cache is usually used to buffer reads and writes to files.
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It is also used to provide the pages which are mapped into userspace
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by a call to mmap.
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For block devices that are memory-like, the page cache pages would be
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unnecessary copies of the original storage. The DAX code removes the
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extra copy by performing reads and writes directly to the storage device.
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For file mappings, the storage device is mapped directly into userspace.
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Usage
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-----
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If you have a block device which supports DAX, you can make a filesystem
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on it as usual. The DAX code currently only supports files with a block
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size equal to your kernel's PAGE_SIZE, so you may need to specify a block
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size when creating the filesystem. When mounting it, use the "-o dax"
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option on the command line or add 'dax' to the options in /etc/fstab.
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Implementation Tips for Block Driver Writers
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--------------------------------------------
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To support DAX in your block driver, implement the 'direct_access'
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block device operation. It is used to translate the sector number
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(expressed in units of 512-byte sectors) to a page frame number (pfn)
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that identifies the physical page for the memory. It also returns a
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kernel virtual address that can be used to access the memory.
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The direct_access method takes a 'size' parameter that indicates the
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number of bytes being requested. The function should return the number
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of bytes that can be contiguously accessed at that offset. It may also
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return a negative errno if an error occurs.
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In order to support this method, the storage must be byte-accessible by
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the CPU at all times. If your device uses paging techniques to expose
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a large amount of memory through a smaller window, then you cannot
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implement direct_access. Equally, if your device can occasionally
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stall the CPU for an extended period, you should also not attempt to
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implement direct_access.
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These block devices may be used for inspiration:
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- axonram: Axon DDR2 device driver
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- brd: RAM backed block device driver
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- dcssblk: s390 dcss block device driver
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Implementation Tips for Filesystem Writers
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------------------------------------------
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Filesystem support consists of
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- adding support to mark inodes as being DAX by setting the S_DAX flag in
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i_flags
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- implementing the direct_IO address space operation, and calling
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dax_do_io() instead of blockdev_direct_IO() if S_DAX is set
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- implementing an mmap file operation for DAX files which sets the
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VM_MIXEDMAP and VM_HUGEPAGE flags on the VMA, and setting the vm_ops to
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include handlers for fault, pmd_fault and page_mkwrite (which should
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probably call dax_fault(), dax_pmd_fault() and dax_mkwrite(), passing the
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appropriate get_block() callback)
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- calling dax_truncate_page() instead of block_truncate_page() for DAX files
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- calling dax_zero_page_range() instead of zero_user() for DAX files
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- ensuring that there is sufficient locking between reads, writes,
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truncates and page faults
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The get_block() callback passed to the DAX functions may return
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uninitialised extents. If it does, it must ensure that simultaneous
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calls to get_block() (for example by a page-fault racing with a read()
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or a write()) work correctly.
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These filesystems may be used for inspiration:
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- ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
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- ext4: the fourth extended filesystem, see Documentation/filesystems/ext4.txt
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Shortcomings
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------------
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Even if the kernel or its modules are stored on a filesystem that supports
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DAX on a block device that supports DAX, they will still be copied into RAM.
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The DAX code does not work correctly on architectures which have virtually
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mapped caches such as ARM, MIPS and SPARC.
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Calling get_user_pages() on a range of user memory that has been mmaped
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from a DAX file will fail as there are no 'struct page' to describe
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those pages. This problem is being worked on. That means that O_DIRECT
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reads/writes to those memory ranges from a non-DAX file will fail (note
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that O_DIRECT reads/writes _of a DAX file_ do work, it is the memory
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that is being accessed that is key here). Other things that will not
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work include RDMA, sendfile() and splice().
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