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

[Mateusz Guzik]

On Thu, Dec 17, 2015 at 02:33:46PM -0800, Don Lewis wrote:
> 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.
> 

Not sure I follow, can you rephrase/elaborate?

> > 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.
> 

Well I had such separate bitmaps in mind with addition of a combined
"the id is in use bitmap". This would make the common case of finding a
new pid reasonably fast. Access to all bitmaps would be protected with
proctree lock, which matches current locking scheme anyway.

-- 
Mateusz Guzik <mjguzik gmail.com>