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

[Andrew Morton]

Roland Dreier <roland at topspin.com> wrote:
>
>     Libor>   Do you mean that the set/clear parameters to do_mlock()
>     Libor> are the actual flags which are set/cleared by the caller? 
>     Libor> Also, the issue remains that the flags are not reference
>     Libor> counted which is a problem if you are dealing with
>     Libor> overlapping memory region, or even if one region ends and
>     Libor> another begins on the same page. Since the desire is to be
>     Libor> able to pin any memory that a user can malloc this is a
>     Libor> likely scenario.
> 
> Good point... we need to figure out how to handle:
> 
>     a) app registers 0x0000 through 0x17ff
>     b) app registers 0x1800 through 0x2fff
>     c) app unregisters 0x0000 through 0x17ff
>     d) the page at 0x1000 must stay pinned

The userspace library should be able to track the tree and the overlaps,
etc.  Things might become interesting when the memory is MAP_SHARED
pagecache and multiple independent processes are involved, although I guess
that'd work OK.

But afaict the problem wherein part of a page needs VM_DONTCOPY and the
other part does not cannot be solved.