Re: fork_findpid() - Fatal trap 12: page fault while in kernel mode

On 17 Dec, Mateusz Guzik wrote:
> On Thu, Dec 17, 2015 at 11:48:08AM -0800, Don Lewis wrote:
>> On 17 Dec, Konstantin Belousov wrote:
>> > On Wed, Dec 16, 2015 at 11:08:02AM -0800, Don Lewis wrote:
>> >> I used to have a patch the deferred linking the new process into
>> >> proctree/allproc until it was fully formed.  The motivation was to get
>> >> rid of all of the PRS_NEW stuff scattered around the source.
>> >> Unfortunately the patch bit-rotted and I'm pretty sure that I lost it.
>> > 
>> > I had similar tought for a second as one of the possibilities to fix the
>> > issue, but rejected it outright due to the way the pid allocator works.
>> > The loop which faulted is the allocator, it depends on the new pid being
>> > linked early to detect the duplicated alloc.
>> > 
>> > What you wrote could be done, but this restructuring requires the separate
>> > pid allocator, and probably it must repeat all quirks and subtle behaviour
>> > of the current algorithm.  But I do not object, PRS_NEW is a trouble
>> > on its own.
>> 
>> I don't think it requires any changes to the allocater.  It should only
>> be necessary to delay the call to fork_findpid() until we are ready to
>> link the new proc into allproc.  Basically, drop the locks at the
>> beginning of do_fork(), then grab them again somewhere near the end
>> (probably where we are currently mark the process as PRS_NORMAL) and
>> move the call to fork_findpid(), the p2->p_pid assignment, and the list
>> manipulation code to a location after that.
>> 
>> It's probably not quite that simple though ...
> 
> That would mean you would need to be able to deconstruct the process
> because you cannot guarantee there are any pids left, which may or may
> not be easily doable.

It doesn't look like we handle that properly in the current code.  I
think fork_findpid() will loop forever.  It shouldn't be possible if
maxproc < pid_max / 3, or maybe pid_max / 2.  It might be a good idea to
enforce this.

> The current method is going to bite us performance-wise anyway and an
> allocater which does not require a walk over the tree is necessary in
> the long run. Seems like a bitmap (or a bunch of bitmaps) is the way to
> go here.

I think that separate bitmaps for process, process group, and session
ids would be needed.  It would waste some space, but it's probably more
efficent to use a byte array and store all the bits for the pid
together.

> Meanwhile one can add a special process permanently in PRS_NEW state and
> poisoned pointers in debug kernels to help ensuring that all loops
> handle the case.
> 
> Not signing up for any of this work though.