Re: 2.6.13-rc3: cache flush missing from somewhere

From: David S. Miller
Date: Sat Jul 30 2005 - 14:42:24 EST

From: Russell King <rmk+lkml@xxxxxxxxxxxxxxxx>
Date: Fri, 29 Jul 2005 16:13:43 +0100

> My current patch to get this working is below. The only thing which
> really seems to fix the issue is the __flush_dcache_page call in
> read_pages() - if I remove this, I get spurious segfaults and illegal
> instruction faults.

If one cpu stores, does it get picked up in the other cpu's I-cache?
If not, you cannot use the lazy dcache flushing method, and in fact
you must broadcast the flush on all processors.

> If I make flush_dcache_page() non-lazy, this also fixes it, but this
> is not desirable. The problem also goes away if I disable the write
> allocate cache mode.


> If I call __flush_dcache_page() from update_mmu_cache() (iow, always
> ensure that we have I/D coherency when the page is mapped into user
> space) the effect is the same - I see random faults.

You have to do the flush on the processor that does the store,
at a minimum. But if other cpus have no way to "notice" stores
on other cpus in their I-cache, this alone is not sufficient.

Based upon your report, I strongly suspect that remote I-caches
do not see cpu local stores when the cache is in write allocate
mode. If this is true, it's a horrible design decision for an
SMP system :(

> To me, it feels like there's a path which results in pages mapped into
> user space without update_mmu_cache() being called, but I'm unable to
> find it. Ideas?

Ptrace can make this occur, but that's obviously not what you're
seeing here.
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