Re: [PATCH 1/3] Fix COW D-cache aliasing on fork

[Linus Torvalds <torvalds@osdl.org>]

On Sat, 21 Oct 2006, Nick Piggin wrote:
> > So maybe the COW D$ aliasing patch-series is just the right thing to do. Not
> > worry about D$ at _all_ when doing the actual fork, and only worry about it
> > on an actual COW event. Hmm?
> 
> Well if we have the calls in there, we should at least make them work
> right for the architectures there now. At the moment the flush_cache_mm
> before the copy_page_range wouldn't seem to do anything if you can still
> have threads dirty the cache again through existing TLB entries.
>
> I don't think that flushing on COW is exactly right though, because dirty
> data can remain invisible if you're only doing reads (no write, no flush).

You're right. A virtually indexed cache needs the flush _before_ we return 
from the fork into a new process (since otherwise the dirty data won't be 
visible in the new virtual address space).

So you've convinced me. Flushing at COW time _cannot_ be right, because it 
by definition means that there has been a time when the new process didn't 
see the dirty data in the case of a virtual index. And in the case of a 
physical index it cannot matter.

So I think the right thing to do is to forget about the COW D$ series 
(which probably _hides_ most of the problems in practice, so it "works" 
that way) and instead go with Ralf's last patch that just moves the 
flush_cache_mm() to after the TLB flush.

We do need to have all the architecture people (especially S390, which has 
been very strange in this regard in the past) check that it's ok. The 
_mappings_ are still valid, so S390 should be able to do the write-back, 
but there may be architectures that would want to do the flush _both_ 
before and after (for performance reasons - if writing out dirty data 
requires a TLB lookup, doing most fo the writeback before is probably a 
better thing, and then we can do a _second_ writeback after the flush to 
close the race with some other thread dirtying the pages before the TLB 
was marked read-only).

I added linux-arch and Martin Schwidefsky (s390) to the Cc:.

Guys, in case you missed the earlier discussion: there's a suggested patch 
by Ralf Baechle on linux-kernel (but it does just the "flush after" 
version, not the "perhaps we need it both before and after" thing I 
theorise about above). Message-ID: 20061020160538.GB18649@linux-mips.org.

		Linus
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