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
patch.
> * 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.
Regards,
Alan
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