Page faults in managed dataspaces

[Denis Huber]

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.


Kind regards,
Denis


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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