On Nov 8, 2010, at 11:25 PM, Masao Uebayashi wrote: > http://uebayasi.dyndns.org/~uebayasi/tmp/uebayasi-xip-20101109.txt
Besides the churn (and there is a lot of it), I think my fundamental problem with this incarnation of XIP is that it took a wrong approach. It has tried to fit itself under uvm_vnodeops and I think that's a fatal flaw by requiring invasive changes to contort to that decision. Instead, XIP should have its own pager ops uvm_xipops and vnodes should be set to use that in vnalloc/insmntque which is easily done since you can just check for MNT_XIP in the passed mount point. xipops would a pgo_fault routine to handle the entering the corresponding PA for the faulting VA using similar logic to the genfs_getpages_xip in your patch. This avoid the entire issue about vm_pages entirely since pgo_fault will be calling pmap_enter directly with the correct paddr_t. We will need to add a PMAP_CACHE flag as well. This leaves the issue of how to deal with copy-on-write (COW) page faults. I think the best way to deal with is for pgo_fault to return a specific error (EROFS seems appropriate) and let UVM deal with it. However UVM doesn't know where the source data exists so we will need to add a int (*pgo_copy_page)(struct uvm_object *uobj, voff_t offset, paddr_t pa) op to pagerops which copy one page of data from uvm_object to the specified pa. This would do what we would normally use pmap_copy_page but we can't since we don't know the source pa and it's not a managed page. I think with this approach most of churn goes away and there is minimal changes to the rest of NetBSD.
