How to find all the tasks with a Sys V shm?

How to find all the tasks with a Sys V shm?

[Kamran Karimi]

Hello all,

In 2.2 kernels one could access the vma segments of a Sys V shared memory 
(shm) through:
shp->attaches. In 2.6 kernels a shm is managed through a file. The question 
is: given a Sys V  key value, how can we find all the tasks that have 
attached the corresponding shm?

We can do the following (with error checking ignored):

    id = ipc_findkey(&shm_ids, key);
    shp = shm_lock(id);

    tsk = find_task_by_pid(shp->shm_cprid);

    vma = tsk->mm->mmap;
    while(vma) {
                 if(vma->vm_file == shp->shm_file)
                  break;
                 vma = vma->vm_next;
    }
    shm_unlock(shp);

But here we only get the vma of the task that created the shm, and not any 
other task who has attached it.

How can we find all the vma structures that represent a shm?

-Kamran

Re: How to find all the tasks with a Sys V shm?

[Hugh Dickins]

On Tue, 7 Mar 2006, Kamran Karimi wrote:
> 
> id = ipc_findkey(&shm_ids, key);
> shp = shm_lock(id);
> ...
> 
> But here we only get the vma of the task that created the shm, and not any
> other task who has attached it.
> 
> How can we find all the vma structures that represent a shm?

The simplest example to follow would be reset_vma_truncate_counts() in
mm/memory.c: shp->shm_file->f_mapping gives you the struct address_space *
mapping to pass to vma_prio_tree_foreach().  Ignore the i_mmap_nonlinear
lines, you can't get nonlinear shm (because shm_vm_ops lacks .populate).
You'll need to spin_lock(&mapping->i_mmap_lock) first, spin_unlock after.

Hugh