mkdaxfs fails with uncached-minus mode error (kernel only supports write-back for CXL device)

[haironghu]

I attempted to mount the physical address of a CXL device using mkdaxfs, but encountered an error. The error details are as follows:

The CXL device is configured as a Device-DAX device. The physical address range of the DAX device is:

The dmesg output is as follows:

By analyzing the dmesg logs and cross-referencing the source code, I identified the root cause of the failure when using mkdaxfs to mount the physical address of a DAX device:
The uncached-minus memory mode is not supported by the DAX device's underlying memory hardware and kernel rules. The kernel can only allocate the write-back mode, but mkdaxfs explicitly requests the unsupported uncached-minus mode in strict mode, which triggers the error.

Could you provide guidance on how to resolve this error? Does mkdaxfs support CXL devices? @congwang-mk

[congwang-mk]

I think you are right, I have fixed the mkdaxfs tool accordingly. Thanks for your suggestion. Please try it again, I don't have a DAX device.

[haironghu]

I have two further questions:
The -b option (specify block size) does not work in the current main branch code？
Is the daxfs-branch tool no longer supported/maintained?

[congwang-mk]

'-b' was for '--branching', not for block size. It has been removed together when we removed the branching support. Same for daxfs-branch tool.

If you are interested in branching, you could still use BranchFS (https://github.com/multikernel/branchfs) on top of DAXFS.