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SubjectRe: VM subsystem bug in 2.4.0 ?


On 9 Jan 2001, Christoph Rohland wrote:

> Linus Torvalds <torvalds@transmeta.com> writes:
> >
> > Note that this would be solved very cleanly if the SHM code would use the
> > "VM_LOCKED" flag, and actually lock the pages in the VM, instead of trying
> > to lock them down for writepage().
>
> here comes the patch. (lightly tested)

I'd really like an opinion on whether this is truly legal or not? After
all, it does change the behaviour to mean "pages are locked only if they
have been mapped into virtual memory". Which is not what it used to mean.

Arguably the new semantics are perfectly valid semantics on their own, but
I'm not sure they are acceptable.

In contrast, the PG_realdirty approach would give the old behaviour of
truly locked-down shm segments, with not significantly different
complexity behaviour.

What do other UNIXes do for shm_lock()?

The Linux man-page explicitly states for SHM_LOCK that

The user must fault in any pages that are required to be present
after locking is enabled.

which kind of implies to me that the VM_LOCKED implementation is ok.
HOWEVER, looking at the HP-UX man-page, for example, certainly implies
that the PG_realdirty approach is the correct one. The IRIX man-pages in
contrast say

Locking occurs per address space;
multiple processes or sprocs mapping the area at different
addresses each need to issue the lock (this is primarily an
issue with the per-process page tables).

which again implies that they've done something akin to a VM_LOCKED
implementation.

Does anybody have any better pointers, ideas, or opinions?

Linus

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