Re: [PATCH] ppc64: Fix possible race with set_pte on a present PTE

[Linus Torvalds]

On Wed, 26 May 2004, Benjamin Herrenschmidt wrote:
> 
> Well, just setting one of those 2 bits doesn't require a hash table
> invalidate as long as nothing else changes.

I'm starting to doubt this, because:

> We do dirty by mapping r/o in the hash table, and accessed on hash
> faults (our clear_young triggers a flush). So just setting those bits
> in the linux PTE without touching the hash table is fine, we'll just
> possibly take an extra fault on the next write or access, but that
> might not be much slower than going to the hash update the permissions
> directly.

But if we don't update the hash tables, how will the TLB entry _ever_ say 
that the page is writable? So we won't take just _one_ extra fault on the 
next write, we'll _keep_ taking them, since the hash tables will continue 
to claim that the page is read-only, even if the linux sw page tables say 
it is writable.

So I think the code needs to invalidate the hash after having updated the 
pte. No?

		Linus
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