hi thor, your tls-maxphys branch appears to assume that a given device's maxphys is constant, but consider the case of a software "device" like a logical volume from LVM. one very nice feature of a volume manager that I used to work with (veritas vxvm) is that it can migrate a volume to new storage on the fly, while the data in it is being read and written. with such a virtual device, I assume that intention would be that the maxphys of the volume would be the same as the maxphys of the underlying physical device, but the maxphys of the underlying physical device could be different before and after a volume migration, so that doesn't fit very well with the assumption that a device's maxphys doesn't change.
caching "the" device maxphys in a file system's "struct mount" is also problematic since a file system may directly access multiple underlying devices, as ZFS does, and those devices may each have a different maxphys. could you think about how to enhance the new maxphys design to accomodate these cases? -Chuck