[openib-general] Re: [PATCH][RFC][0/4] InfiniBand userspace verbs implementation

[Arjan van de Ven]

On Mon, 2005-04-18 at 11:09 -0500, Timur Tabi wrote:
> Roland Dreier wrote:
> >     Troy> How is memory pinning handled? (I haven't had time to read
> >     Troy> all the code, so please excuse my ignorance of something
> >     Troy> obvious).
> > 
> > The userspace library calls mlock() and then the kernel does
> > get_user_pages().
> 
> Why do you call mlock() and get_user_pages()?  In our code, we only call mlock(), and the 
> memory is pinned. 

this is a myth; linux is free to move the page about in physical memory
even if it's mlock()ed!!

And even then, the user can munlock the memory from another thread etc
etc. Not a good idea.

get_user_pages() is used from AIO and other parts of the kernel for
similar purposes and in fact is designed for it, so it better work. If
it has bugs those should be fixed, not worked around!