Messages in this thread | | | Date | Fri, 10 Feb 2006 15:15:02 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: msync() behaviour broken for MS_ASYNC, revert patch? |
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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.
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