VM wiring fixed
Brian Fundakowski Feldman
green at FreeBSD.org
Wed Apr 28 20:05:13 PDT 2004
Alan Cox <alc at cs.rice.edu> wrote:
> 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?
Yeah, same deal, except since we're optimizing by only clearing/
reinitializing some fields for the vmspace-zone objects, it makes a little
more sense IMO to have it in the vmspace_dofree().
> > * 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.
I changed kmem_free() to do an unwire since it is the interface for
unmapping wired memory, as opposed to kmem_free_wakeup().
> > * 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.
I didn't realize the problem was so old!
> > * 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.
I've done it the way I explained to preserve layering of vmspace -> map ->
entry -> page as best I knew how. The kmem_free() implementation doesn't
know if the entries are MAP_ENTRY_NOFAULT or not, so I pushed the
intelligence into the wiring routines as to what that means.
> > * 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.
How can it be done, then? The vslock() function is called with the
expectation that the memory it references is both "wired" and "held". I
can't see any other to do it -- there is no refcount on entries. Since
vslock() is the only interface for system-wiring arbitrary memory, there
will always be a matching call to vsunlock() which causes the system unwire
to happen and the waiting vm_map_delete() to wake up.
> > * 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.
Would you rather see this or an "OOM-wired-killer"?
> > * 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.
Thanks!
--
Brian Fundakowski Feldman \'[ FreeBSD ]''''''''''\
<> green at FreeBSD.org \ The Power to Serve! \
Opinions expressed are my own. \,,,,,,,,,,,,,,,,,,,,,,\
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