On Fri, Oct 20, 2006 at 02:12:11PM -0700, Linus Torvalds wrote:
> > Well, looking at do_wp_page() I'm now quite concerned about ARM and COW.
> > I can't see how this code could _possibly_ work with a virtually indexed
> > cache as it stands. Yet, the kernel does appear to work.
>
> It really shouldn't need any extra code, exactly because by the time it
> hits any page-fault, the caches had better be in sync with the physical
> page contents _anyway_ (yes, being virtual, the caches will _duplicate_
> the contents, but since the pages are read-only, that aliasing should be
> perfectly fine).
Until yesterday I also thought multiple read-only copies wouldn't do any
harm. Well, until I learned about the wonderful behaviour of the PA8800
caches. PA8800 has VIPT primary caches, PIPT secondary caches. And the
sinister part - caches are exclusive, that is a cacheline is either in
L1 or L2 but never in both and can migrate between L1 and L2. Now
onsider the following scenario:
o physical address P is mapped to two aliasing addresses V1 and V2
o a load from V1 results in a clean line in L1 caching P at index V1.
o a store to V2 results in a clean line in L1 caching P at index V2.
o the line at V2 is getting written back to memory.
o a victim replacement of the line at V1 results in the _clean_ line
migrating back from L1 to L2.
-> another read from V2 will return stale data.
As consequence flush_cache_mm() on PA (or at least PA8800) currently blows
away the entire cache, as Kyle McMartin just told me. The whole 1.5MB L1
and 32MB of L2 making fork an ultraheavy operation.
> It's just that we weren't quite careful enough at that time (and even
> then, that would only matter for some really really unlikely and strange
> situations that only happen when you fork() from a _threaded_ environment,
> so it shouldn't be anything you'd notice under normal load).
>
> I think.
The flush is there since a very long time. I have it in my tree since
~ 2.1.36 and I get the feeling anybody every has been seriously revisited
the issue since.
Ralf
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