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DAX may be limited in some specific situation. When the number of usable DAX windows is under watermark, the recalim routine will be triggered to reclaim some DAX windows. It may have a negative impact on the performance, since some processes may need to wait for DAX windows to be recalimed and reused then. To mitigate the performance degradation, the overall DAX window need to be expanded larger. However, simply expanding the DAX window may not be a good deal in some scenario. To maintain one DAX window chunk (i.e., 2MB in size), 32KB (512 * 64 bytes) memory footprint will be consumed for page descriptors inside guest, which is greater than the memory footprint if it uses guest page cache when DAX disabled. Thus it'd better disable DAX for those files smaller than 32KB, to reduce the demand for DAX window and thus avoid the unworthy memory overhead. Per inode DAX feature is introduced to address this issue, by offering a finer grained control for dax to users, trying to achieve a balance between performance and memory overhead. The FUSE_ATTR_DAX flag in FUSE_LOOKUP reply is used to indicate whether DAX should be enabled or not for corresponding file. Currently the state whether DAX is enabled or not for the file is initialized only when inode is instantiated. Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com> Reviewed-by: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.