Re: [PATCH] New phys_addr() syscall

Raul Miller (rdm@test.legislate.com)
Tue, 21 Jul 1998 04:48:57 -0400

Richard Gooch <Richard.Gooch@atnf.CSIRO.AU> wrote:
> Raul Miller writes:
> > Albert D. Cahalan <acahalan@cs.uml.edu> wrote:
> > > There were plans to make mlock() available to normal users for
> > > cryptographic purposes. There would be a quota to protect the
> > > machine. If a user (or group of users) can get 1/32 of the pages
> > > below 16 MB, then the system can not allocate 128 kB for DMA.
> >
> > You'll eventually want to allow memory locked with this variation of
> > mlock to be migrated out of the DMA region (to deal with fragmentation
> > issues).
>
> That may present an unacceptable cost to a RT application. Copying a
> page can easily take 100 microseconds. If Linux ever has the ability
> to pin a RT process to a CPU, that process should never be delayed if
> it doesn't ask to I/O (assuming it's mlock(2)ed all it's pages).

I thought you needed special priviledge to do RT? I thought Albert
Cahalan was speaking of allowing unpriviledged mlocks?

> Moving to another physical page is contrary to the spirit of mlock(2),
> IMO.

I suppose an alternative, at least for the unpriviledged mlock,
would be to never allow memory from the low 16M (except on machines
with 16M or less).

-- 
Raul


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