LU-19179 removed RHEL 7 kernel support and along with it, removed the copy of XArray from the Lustre tree and relied on the in-kernel XArray implementation. It also added a configure test that checks for the presence of CONFIG_XARRAY_MULTI, so that the build fails, just in case Lustre is built against a kernel without XArray support.

CONFIG_XARRAY_MULTI doesn't denote core XArray support, though, but the presence of an XArray feature for supporting entries, which occupy multiple consecutive indices in the XArray; it was assumed that CONFIG_XARRAY_MULTI would normally always be set, as CONFIG_ZONE_DEVICE, which is also expected to be normally set (and also CONFIG_TRANSPARENT_HUGEPAGE) select CONFIG_XARRAY_MULTI. It seems to be possible in practice, to not have either of these configuration options set in some SLES 15 SP2/SP3 kernels (probably with a custom configuration) that we have built against and this should also be possible on kernels of any currently supported distributions and on vanilla kernels.

The functionality provided by CONFIG_XARRAY_MULTI isn't used by Lustre and core XArray support should be sufficient, so this shouldn't be a requirement. We could reinstate the previous configure check that verified the presence of XArray via a compilation test, to address this issue.