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Diffstat (limited to 'Documentation/filesystems/dax.rst')
| -rw-r--r-- | Documentation/filesystems/dax.rst | 28 |
1 files changed, 21 insertions, 7 deletions
diff --git a/Documentation/filesystems/dax.rst b/Documentation/filesystems/dax.rst index 9a1b8fd9e82b..5b283f3d1eb1 100644 --- a/Documentation/filesystems/dax.rst +++ b/Documentation/filesystems/dax.rst @@ -23,11 +23,11 @@ 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. +Currently 5 filesystems support `DAX`: ext2, ext4, xfs, virtiofs and erofs. +Enabling `DAX` on them is different. -Enabling DAX on ext2 --------------------- +Enabling DAX on ext2 and erofs +------------------------------ 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 @@ -168,6 +168,22 @@ if the underlying media does not support dax and/or the filesystem is overridden with a mount option. +Enabling DAX on virtiofs +---------------------------- +The semantic of DAX on virtiofs is basically equal to that on ext4 and xfs, +except that when '-o dax=inode' is specified, virtiofs client derives the hint +whether DAX shall be enabled or not from virtiofs server through FUSE protocol, +rather than the persistent `FS_XFLAG_DAX` flag. That is, whether DAX shall be +enabled or not is completely determined by virtiofs server, while virtiofs +server itself may deploy various algorithm making this decision, e.g. depending +on the persistent `FS_XFLAG_DAX` flag on the host. + +It is still supported to set or clear persistent `FS_XFLAG_DAX` flag inside +guest, but it is not guaranteed that DAX will be enabled or disabled for +corresponding file then. Users inside guest still need to call statx(2) and +check the statx flag `STATX_ATTR_DAX` to see if DAX is enabled for this file. + + Implementation Tips for Block Driver Writers -------------------------------------------- @@ -190,8 +206,6 @@ 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 @@ -275,7 +289,7 @@ The DAX code does not work correctly on architectures which have virtually mapped caches such as ARM, MIPS and SPARC. Calling :c:func:`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 +mmapped 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 |