VM wiring fixed

Alan Cox alc at cs.rice.edu
Wed Apr 28 19:24:03 PDT 2004

On Wed, Apr 28, 2004 at 08:08:17PM -0400, Brian Fundakowski Feldman wrote:
> There are several severe wiring bugs in -CURRENT that I believe I have fixed.
> Please test/review as appropriate if you're affected by any of them.  This 
> is a superset of the previous patch which just mostly-fixed mlockall(2).
> * MAP_FUTUREWIRE was not unset in vmspace_dofree(), causing the next process 
>   to use that vmspace to wire all of its memory.

Would setting flags to 0 in vmspace_alloc() accomplish the same?

> * kmem_*() calls either called vm_map_wire() or set MAP_ENTRY_NOFAULT, but
>   kmem_free() did not undo the vm_map_wire() calls.

I don't understand this comment.  kmem_free() calls vm_map_remove(), which
calls vm_map_delete(), which calls vm_map_entry_unwire(), etc.

However, I don't see any change corresponding to this statement in your

> * vm_fault_unlock() could not unwire pages that were not in the pmap already,
>   leaking them permanently.

By definition a wired mapping is in the pmap.  A wired page can, however,
be mapped by a non-wired mapping.  Thus, a non-wired mapping of a wired
page could be absent from the pmap.

Problems such as PR/29915 are a result of vm_fault_unwire() not understanding
fake pages created by the device pager.   Pmap_extract() followed by
PHYS_TO_VM_PAGE() does not result in a pointer to the fake page that was
wired.  Thus, the panic on unexpected wiring count.

Your patch to vm_fault_unwire() does look like a correct fix to this issue.

> * vm_map_{un,}wire() did not keep track of wirings as they should.  User
>   wirings are separate from system wirings, and there can be exactly one.
>   There can be unlimited system wirings, but wired_count will remain at
>   zero for map entries that are allocated as MAP_ENTRY_NOFAULT.

MAP_ENTRY_NOFAULT means that the vm_map_entry has no backing vm_object and
that you should never fault on it, which would create a backing object.
vm_map_{un,}wire(), however, deal with map entries that do have backing
objects.  I'm not quite sure why you chose to introduce MAP_ENTRY_NOFAULT
into the implementation of vm_map_{un,}wire().

I'll have to look at this more carefully.

> * vslock()/vsunlock() did not both use VM_MAP_WIRE_SYSTEM as they must;
>   I believe this resulted in more wiring leaks.
> * vm_map_delete() did not wait for all wirings (except one user wiring) to
>   drain, so vslock() guaranteed nothing.

It is not vm_map_delete()'s responsibility to wait for all wirings to be
drained.  It is only responsible for a single wired mapping. 

> * The condition in vm_fault() where all pages have been exhausted is easy to
>   deadlock, but was impossible to recover from.  The OOM killer works only
>   when all memory has been used, not all wired memory.  However, now it is
>   possible to kill offending processes with SIGKILL instead of the
>   vm_fault() in trap_pfault() looping forever.
> * The init(8) program should really be using mlockall(2) so that it can kill
>   off processes hogging all the wired memory.  However, I have not fixed this
>   because in such case, e.g. while I would like for Ctrl+Alt+Del to work,
>   init(8) may actually need to allocate and wire new pages itself to keep
>   running.  I think a way to fix this is to conditionalize the
>   vm_page_count_severe() condition on p->p_pid != 1 so that just like the
>   REAL system processes, it can bring the page count lower than "severe".
> I haven't tested it out on SMP yet, but on UP the latest changes don't seem 
> to have any negative effects.  All wired memory leaks appear to be gone and
> although init(8) probably can't do it, I can enter DDB and "kill 9 <wirehog>"
> to take the machine out of an all-wired deadlock.  See patch at URL:
> <http://green.homeunix.org/~green/vm-wiring.patch>

In summary, consider this a positive review for the MAP_FUTUREWIRE and
the vm_fault_unwire() fixes.


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