On Wed, 1 Mar 2006, Kamran Karimi wrote:
>
> Thank you Hugh for the reply. Last time I used VM_SHM was in 2.2.x kernels.
> I have a programme called DIPC which makes System V shared memory segments
> (and also messages and semaphores) work over a network.
>
> In the arch/xyz/mm/fault.c file, it checks the VM_SHM flag and then calls
> its logic. As a substitute I've been trying this ad-hoc code to see if a vma
> represents a Sys V shm:
>
> file = vma->vm_file;
> if(file && (file->f_dentry) && (file->f_dentry->d_inode) &&
> (id = file->f_dentry->d_inode->i_ino)) {
> shp = shm_lock(id);
> if(shp == NULL)
> return 0; // not a Sys V shm
> }
> else return 0; // not a Sys V shm
>
> But the kernel hangs with an invalid-pointer error message. Any suggestions?
It's not obvious to me why the kernel would hang with an invalid pointer
error message there: ipc_lock appears to have good safety against being
passed a random id. Perhaps the invalid pointer message comes from
other code you've not shown (for example, I hope you shm_unlock(shp)
and return 1 when shm_lock succeeds), or perhaps I'm misreading.
But what you're doing there looks entirely weird and meaningless to me:
if shm_lock happens to succeed or fail on the inode number of some file
on some filesystem, that tells you nothing about whether that file is
SysV shm or not. Ah, I see ipc/shm.c saves id in i_ino: so if you're
dealing with a SysV shm file, then indeed that ought to tell whether
you're dealing with a SysV shm file - but that hasn't helped much!
Since you're already patching base kernel source (you mention
arch/xyz/mm/fault.c), why don't you just patch your own VM_SYSVSHM
into include/linux/mm.h, and set it on the vma in ipc/shm.c?
Hugh
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