just skip that page. this situation can arise legitimately when a file
with a wired mapping is truncated so that a wired page is no longer
part of the file.
from VM_FAULT_WIRE in that when the pages being wired are faulted in,
the simulated fault is at the maximum protection allowed for the mapping
instead of the current protection. use this in uvm_map_pageable{,_all}()
to fix the problem where writing via ptrace() to shared libraries that
are also mapped with wired mappings in another process causes a
diagnostic panic when the wired mapping is removed.
this is a really obscure problem so it deserves some more explanation.
ptrace() writing to another process ends up down in uvm_map_extract(),
which for MAP_PRIVATE mappings (such as shared libraries) will cause
the amap to be copied or created. then the amap is made shared
(ie. the AMAP_SHARED flag is set) between the kernel and the ptrace()d
process so that the kernel can modify pages in the amap and have the
ptrace()d process see the changes. then when the page being modified
is actually faulted on, the object pages (from the shared library vnode)
is copied to a new anon page and inserted into the shared amap.
to make all the processes sharing the amap actually see the new anon
page instead of the vnode page that was there before, we need to
invalidate all the pmap-level mappings of the vnode page in the pmaps
of the processes sharing the amap, but we don't have a good way of
doing this. the amap doesn't keep track of the vm_maps which map it.
so all we can do at this point is to remove all the mappings of the
page with pmap_page_protect(), but this has the unfortunate side-effect
of removing wired mappings as well. removing wired mappings with
pmap_page_protect() is a legitimate operation, it can happen when a file
with a wired mapping is truncated. so the pmap has no way of knowing
whether a request to remove a wired mapping is normal or when it's due to
this weird situation. so the pmap has to remove the weird mapping.
the process being ptrace()d goes away and life continues. then,
much later when we go to unwire or remove the wired vm_map mapping,
we discover that the pmap mapping has been removed when it should
still be there, and we panic.
so where did we go wrong? the problem is that we don't have any way
to update just the pmap mappings that need to be updated in this
scenario. we could invent a mechanism to do this, but that is much
more complicated than this change and it doesn't seem like the right
way to go in the long run either.
the real underlying problem here is that wired pmap mappings just
aren't a good concept. one of the original properties of the pmap
design was supposed to be that all the information in the pmap could
be thrown away at any time and the VM system could regenerate it all
through fault processing, but wired pmap mappings don't allow that.
a better design for UVM would not require wired pmap mappings,
and Chuck C. and I are talking about this, but it won't be done
anytime soon, so this change will do for now.
this change has the effect of causing MAP_PRIVATE mappings to be
copied to anonymous memory when they are mlock()d, so that uvm_fault()
doesn't need to copy these pages later when called from ptrace(), thus
avoiding the call to pmap_page_protect() and the panic that results
from this when the mlock()d region is unlocked or freed. note that
this change doesn't help the case where the wired mapping is MAP_SHARED.
discussed at great length with Chuck Cranor.
fixes PRs 10363, 12554, 12604, 13041, 13487, 14580 and 14853.
