Re: USB mass storage and ARM cache coherency

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2010-02-17 at 20:44 +0000, Russell King - ARM Linux wrote:
> No, because that'd probably bugger up the Sparc64 method of delaying
> flush_dcache_page.
> 
> This method works as follows:
> 
> - a page cache page is allocated - this has PG_arch_1 clear.
> 
> - IO happens on it and it's placed into the page cache.  PG_arch_1 is
>   still clear.
> 
> - someone calls read()/write() which accesses the page.  The generic
>   file IO layers call flush_dcache_page() in response to
> read()/write()
>   fs calls.  flush_dcache_page() spots that the page is not yet mapped
>   into userspace, and sets PG_arch_1 to mark the fact that the kernel
>   mapping is dirty.
> 
> - when someone maps the page, we check PG_arch_1 in update_mmu_cache.
>   If PG_arch_1 is set, we flush the kernel mapping.
> 
> Clearly, if we go around having drivers clearing PG_arch_1, this is
> going to break horribly. 

Ok, you do things very differently than us on ppc then. We clear
PG_arch_1 in flush_dcache_page(), and we set it when the page has been
cache cleaned for execution.

We assume that anybody that dirties a page in the kernel will call
flush_dcache_page() which removes our PG_arch_1 bit thus marking the
page "dirty".

Note that from experience, doing the check & flushes in
update_mmu_cache() is racy on SMP. At least for I$/D$, we have the case
where processor one does set_pte followed by update_mmu_cache(). The
later isn't done yet but processor 2 sees the PTE now and starts using
it, cache hasn't been fully flushed yet. You may avoid that race in some
ways, but on ppc, I've stopped using that.

I now do things directly in set_pte_at(). In fact, that's why I want
your patch to change update_mmu_cache() to take a PTE pointer :-) Since
my set_pte_at() can now remove the _PAGE_EXEC bit, I need
update_mmu_cache() to re-read the PTE before it updates the hash table
or TLB.

Cheers,
Ben.