Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

Blaisorblade
2007-03-08T12:39:34Z

On Wednesday 07 March 2007 10:44, Bill Irwin wrote:
> On Wed, Mar 07, 2007 at 10:28:21AM +0100, Nick Piggin wrote:
> > Depending on whether anyone wants it, and what features they want, we
> > could emulate the old syscall, and make a new restricted one which is
> > much less intrusive.
> > For example, if we can operate only on MAP_ANONYMOUS memory and specify
> > that nonlinear mappings effectively mlock the pages, then we can get
> > rid of all the objrmap and unmap_mapping_range handling, forget about
> > the writeout and msync problems...
>
> Anonymous-only would make it a doorstop for Oracle, since its entire
> motive for using it is to window into objects larger than user virtual
> address spaces (this likely also applies to UML, though they should
> really chime in to confirm).

We need it for shared file mappings (for tmpfs only).

Our scenario is:
RAM is implemented through a shared mapped file, kept on tmpfs (except by dumb
users); various processes share an fd for this file (it's opened and
immediately deleted).

We maintain page tables in x86 style, and TLB flush is implemented through
mmap()/munmap()/mprotect().

Having a VMA per each 4K is not the intended VMA usage: for instance, the
default /proc/sys/vm/max_map_count (64K) is saturated by a UML process with
64K * 4K = 256M of resident memory.

> Restrictions to tmpfs and/or ramfs would
> likely be liveable, though I suspect some things might want to do it to
> shm segments (I'll ask about that one).

> There's definitely no need for a
> persistent backing store for the object to be remapped in Oracle's case,
> in any event. It's largely the in-core destination and source of IO, not
> something saved on-disk itself.
>
>
> -- wli

--
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Paolo Giarrusso, aka Blaisorblade
http://www.user-mode-linux.org/~blaisorblade
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