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SubjectRe: msync() behaviour broken for MS_ASYNC, revert patch?


On Fri, 10 Feb 2006, Trond Myklebust wrote:
> >
> > Now, in a _coherent_ environment (like Linux) it should probably be a
> > no-op, since the mapping is always consistent with storage (where
> > "storage" doesn't actyally mean "disk", but the virtual file underneath
> > the mapping).
>
> Hmmm.... When talking about syncing to _permanent_ storage one usually
> is talking about what is actually on the disk.

Ok, in that case Linux has never done what MS_INVALIDATE says, and I doubt
anybody else has either. It's neither sane nor even really doable (you'd
have to yank out everybody _elses_ caches too, not just your own).

So I think that within this context, the "permanent storage" really means
the "file" that is mapped, and doesn't care about whether it has actually
hit the disk yet.

> In any case, we do have non-coherent mmapped environments in Linux (need
> I mention NFS, CIFS, ... ;-)?).

Good point. That's an argument for actually dropping the local page cache
entirely on such a filesystem, since such a filesystem really isn't
fundamentally coherent.

However, that would be some really _nasty_ semantics, because it would
mean that something like NFS would behave very fundamentally differently
than a local filesystem, even if the user only actually uses it on the
local machine and there are no other writers ("..but there _could_ be
other writers that we don't know about").

So I'd have to veto that just on the grounds of trying to keep users sane.

> IIRC msync(MS_INVALIDATE) on Solaris was actually often used by some
> applications to resync the client page cache to the server when using
> odd locking schemes, so I believe this interpretation is a valid one.

I think you're right. Although I would also guess that 99% of the time,
you'd only do that for read-only mappings. Doing the same in the presense
of also doing writes is just asking for getting shot.

Even for read-only mappings, it's actually quite hard to globally flush a
page cache page if somebody else happens to be using it for a read() or
something at exactly the same time.

Linus
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