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When using FUSE DAX with virtiofs, cache coherency is managed by the host. Disk persistence is handled via fsync() and friends, which are passed directly via the FUSE layer to the host. Therefore, there's no need to do dax_writeback_mapping_range(). All that ends up doing is a cache flush operation, which is not caught by KVM and doesn't do much, since the host and guest are already cache-coherent. Since dax_writeback_mapping_range() checks that the inode block size is equal to PAGE_SIZE, this fixes a spurious WARN when virtiofs is used with a mismatched guest PAGE_SIZE and virtiofs backing FS block size (this happens, for example, when it's a tmpfs and the host and guest have a different PAGE_SIZE). FUSE DAX does not require any particular FS block size, since it always performs DAX mappings in aligned 2MiB blocks. See discussion in [1]. [1] https://lore.kernel.org/lkml/20241101-dax-page-size-v1-1-eedbd0c6b08f@asahilina.net/T/#u [SzM: remove the empty callback] Suggested-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Asahi Lina <lina@asahilina.net> Acked-by: Dan Williams <dan.j.williams@intel.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 reStructuredText 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.