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
<Trond.Myklebust@xxxxxxxxxx> wrote:
> 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
cache coherency.

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.

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