Re: still nfs problems [Was: Linux 2.6.37-rc8]
From: Linus Torvalds
Date: Thu Jan 06 2011 - 13:01:32 EST
On Thu, Jan 6, 2011 at 9:47 AM, Trond Myklebust
> Why is this line needed? We're not writing through the virtual mapping.
I haven't looked at the sequence of accesses, but you need to be
_very_ aware that "write-through" is absolutely NOT sufficient for
In cache coherency, you have three options:
- true coherency (eg physically indexed/tagged caches)
- exclusion (eg virtual caches, but with an exclusion guarantee that
guarantees that aliases cannot happen: either by using physical
tagging or by not allowing cases that could cause virtual aliases)
- write-through AND non-cached reads (ie "no caching at all").
You seem to be forgetting the "no cached reads" part. It's not
sufficient to flush after a write - you need to make sure that you
also don't have a cached copy of the alias for the read.
So "We're not writing through the virtual mapping" is NOT a sufficient
excuse. If you're reading through the virtual mapping, you need to
make sure that the virtual mapping is flushed _after_ any writes
through any other mapping and _before_ any reads through the virtual
This is why you really really really generally don't want to have
aliasing. Purely virtual caches are pure crap. Really.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/