[openib-general] Re: [PATCH] Ref count user doorbell pages in mthca

[Roland Dreier]

    Michael> How does one close the context? I thought thats done by
    Michael> closing the file descriptor, isnt that right? And if not
    Michael> - why not?  I think closing the file descriptor will kill
    Michael> the vmas without work in mthca, avoiding this issue
    Michael> altogether.

Closing a file descriptor does not unmap any mmaps done on that FD.

    Michael> For hotswap we'll need a way to find and unmap all uars,
    Michael> anyway, and I think the same shall be done when context
    Michael> is closed by the application. IMO, when the app closes
    Michael> the context, all resources shall go.

That's not the usual semantics of mmap() vs. closing a file.

    Michael> What lock prevents the reference count from going to 0,
    Michael> and driver deciding to kill the uar object, when at the
    Michael> same time vma (by vm_ops) has a pointer to this object
    Michael> and is calling kref_get?

If the reference count is going to 0, then either:

 - there are no vmas left and the file is already closed so we don't
   have to wory about racing with mmap()

or 

 - the context's file descriptor is already be closed, so mmap() can't
   happen.  We're going through vm_ops->close(), which is protected by
   the usual mm locking (page_table_lock?).

 - R.