Page faults in managed dataspaces

Mon Sep 26 11:13:54 CEST 2016

Hi Dennis,

I've looked into your code, and what struck me first was that you use
two threads in your server, which share data in between
(Resource::Client_resources) without synchronization.

I've rewritten your example server to only use one thread in a
state-machine like fashion, have a look here:

https://github.com/skalk/genode-CheckpointRestore-SharedMemory/commit/d9732dcab331cecdfd4fcc5c8948d9ca23d95e84

This way it is thread-safe, simpler (less code), and if you are adapted
to it, it becomes even easier to understand.

Nevertheless, although the possible synchronization problems are
eliminated by design, your described problem remains. I'll have a deeper
look into our attach/detach implementation of managed dataspaces, but I
cannot promise whether this will happen in short time.

Best regards
Stefan

On 09/26/2016 10:44 AM, Sebastian Sumpf wrote:
> Hey Denis,
> 
> On 09/24/2016 06:20 PM, Denis Huber wrote:
>> Dear Genode Community,
>>
>> perhaps the wall of text is a bit discouraging to tackle the problem. 
>> Let me summaries the important facts of the scenario:
>>
>> * Two components 'ckpt' and 'target'
>> * ckpt shares a thread capability of target's main thread
>> * ckpt shares a managed dataspace with target
>>    * this managed dataspace is initially empty
>>
>> target's behaviour:
>> * target periodically reads and writes from/to the managed dataspace
>> * target causes page faults (pf) which are handled by ckpt's pf handler 
>> thread
>>    * pf handler attaches a pre-allocated dataspace to the managed 
>> dataspace and resolves the pf
>>
>> ckpt's behaviour:
>> * ckpt periodically detaches all attached dataspaces from the managed 
>> dataspace
>>
>> Outcome:
>> After two successful cycles (pf->attach->detach -> pf->attach->detach) 
>> the target does not cause a pf, but reads and writes to the managed 
>> dataspace although it is (theoretically) empty.
>>
>> I used Genode 16.05 with a foc_pbxa9 build. Can somebody help me with my 
>> problem? I actually have no idea what could be the problem.
>>
>>
> 
> You are programming against fairly untested grounds here. There still
> might be bugs or corner cases in this line of code. So, someone might
> have to look into things (while we are very busy right now). Your
> problem is reproducible with [4] right?
> 
> By the way, your way of reporting is exceptional, the more information
> and actual test code we have, the better we can debug problems. So,
> please keep it this way, even though we might not read all of it at times ;)
> 
> Regards and if I find the time, I will look into your issue,
> 
> Sebastian
> 
>>
>>
>> On 19.09.2016 15:01, Denis Huber wrote:
>>> Dear Genode Community,
>>>
>>> I want to implement a mechanism to monitor the access of a component to
>>> its address space.
>>>
>>> My idea is to implement a monitoring component which provides managed
>>> dataspaces to a target component. Each managed dataspace has several
>>> designated dataspaces (allocated, but not attached, and with a fixed
>>> location in the managed dataspace). I want to use several dataspaces to
>>> control the access range of the target component.
>>>
>>> Whenever the target component accesses an address in the managed
>>> dataspace, a page fault is triggered, because the managed dataspace has
>>> no dataspaces attached to it. The page fault is caught by a custom page
>>> fault handler. The page fault handler attaches the designated dataspace
>>> into the faulting managed dataspace and resolves the page fault.
>>>
>>> To test my concept I implemented a prototypical system with a monitoring
>>> component (called "ckpt") [1] and a target component [2].
>>>
>>> [1]
>>> https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/b502ffd962a87a5f9f790808b13554d6568f6d0b/src/test/concept_session_rm/server/main.cc
>>> [2]
>>> https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/b502ffd962a87a5f9f790808b13554d6568f6d0b/src/test/concept_session_rm/client/main.cc
>>>
>>> The monitoring component provides a service [3] to receive a Thread
>>> capability to pause the target component before detaching the dataspace
>>> and resume after detaching and to provide a managed dataspace to the client.
>>>
>>> [3]
>>> https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/tree/b502ffd962a87a5f9f790808b13554d6568f6d0b/include/resource_session
>>>
>>> The monitoring component runs a main loop which pauses the client's main
>>> thread and detaches all attached dataspaces from the managed dataspace.
>>> The target component also runs a main loop which prints (reads) a number
>>> from the managed dataspace to the console and increments (writes) it in
>>> the managed dataspaces.
>>>
>>> The run script is found here [4].
>>>
>>> [4]
>>> https://github.com/702nADOS/genode-CheckpointRestore-SharedMemory/blob/b502ffd962a87a5f9f790808b13554d6568f6d0b/run/concept_session_rm.run
>>>
>>> The scenario works for the first 3 iterations of the monitoring
>>> component: Every 4 seconds it detaches the dataspaces from the managed
>>> dataspace and afterwards resolves the page faults by attaching the
>>> dataspaces back. After the 3. iteration, the target component accesses
>>> the theoretically empty managed dataspaces, but does not trigger a page
>>> fault. In fact, it reads and writes to the designated dataspaces as if
>>> it was attached.
>>>
>>> By running the run script I get the following output:
>>> [init -> target] Initialization started
>>> [init -> target] Requesting session to Resource service
>>> [init -> ckpt] Initialization started
>>> [init -> ckpt] Creating page fault handler thread
>>> [init -> ckpt] Announcing Resource service
>>> [init -> target] Sending main thread cap
>>> [init -> target] Requesting dataspace cap
>>> [init -> target] Attaching dataspace cap
>>> [init -> target] Initialization ended
>>> [init -> target] Starting main loop
>>> Genode::Pager_entrypoint::entry()::<lambda(Genode::Pager_object*)>:Could
>>> not resolve pf=6000 ip=10034bc
>>> [init -> ckpt] Initialization ended
>>> [init -> ckpt] Starting main loop
>>> [init -> ckpt] Waiting for page faults
>>> [init -> ckpt] Handling page fault: READ_FAULT pf_addr=0x00000000
>>> [init -> ckpt]   attached sub_ds0 at address 0x00000000
>>> [init -> ckpt] Waiting for page faults
>>> [init -> target] 0
>>> [init -> target] 1
>>> [init -> target] 2
>>> [init -> target] 3
>>> [init -> ckpt] Iteration #0
>>> [init -> ckpt]   valid thread
>>> [init -> ckpt]   detaching sub_ds_cap0
>>> [init -> ckpt]   sub_ds_cap1 already detached
>>> Genode::Pager_entrypoint::entry()::<lambda(Genode::Pager_object*)>:Could
>>> not resolve pf=6000 ip=10034bc
>>> [init -> ckpt] Handling page fault: READ_FAULT pf_addr=0x00000000
>>> [init -> ckpt]   attached sub_ds0 at address 0x00000000
>>> [init -> ckpt] Waiting for page faults
>>> [init -> target] 4
>>> [init -> target] 5
>>> [init -> target] 6
>>> [init -> target] 7
>>> [init -> ckpt] Iteration #1
>>> [init -> ckpt]   valid thread
>>> [init -> ckpt]   detaching sub_ds_cap0
>>> [init -> ckpt]   sub_ds_cap1 already detached
>>> [init -> target] 8
>>> [init -> target] 9
>>> [init -> target] 10
>>> [init -> target] 11
>>> [init -> ckpt] Iteration #2
>>> [init -> ckpt]   valid thread
>>> [init -> ckpt]   sub_ds_cap0 already detached
>>> [init -> ckpt]   sub_ds_cap1 already detached
>>> [init -> target] 12
>>> [init -> target] 13
>>>
>>> As you can see: After "iteration #1" ended, no page fault was caused,
>>> although the target component printed and incremented the integer stored
>>> in the managed dataspace.
>>>
>>> Could it be, that the detach method was not executed correctly?
>>>
>>>
>>> Kind regards
>>> Denis
>>>
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-- 
Stefan Kalkowski
Genode Labs

https://github.com/skalk · http://genode.org/