linux-kernel - Re: [RFC PATCH] ARM: Assume new page cache pages have dirty D-cache

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Message-ID: <1267869622.4280.5.camel@e102109-lin.cambridge.arm.com>
Date:	Sat, 06 Mar 2010 10:00:22 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH] ARM: Assume new page cache pages have dirty D-cache

On Fri, 2010-03-05 at 21:16 +0000, Russell King - ARM Linux wrote:
> On Fri, Mar 05, 2010 at 04:52:40PM +0000, Catalin Marinas wrote:
> > As Ben said, I think we can set PG_dcache_clean in the
> > clear/copy_user_page() functions. My doubt with these functions is the
> > highmem cases where kunmap_atomic() only flushes the D-cache in one
> > situation, the other just calling kunmap_high() which doesn't seem to do
> > anything to the caches.
> 
> In which case you're totally missing the point with these functions.
> The copy_user_page and clear_user_page functions specifically do tricks
> to ensure that they can avoid additional cache maintainence - or any
> cache maintainence at all.
> 
> For instance, on aliasing VIPT, they will map the user page in using
> the same colour as the ultimate userspace address, ensuring that any
> cache lines created will be visible to the userspace application.

I was more thinking for the non-aliasing VIPT case where we could defer
the flushing until update_mmu_cache(). But I'm fine with just setting
PG_dcache_clean in these functions to avoid checks on non-aliasing vs.
aliasing VIPT.

> So what kunmap_atomic() does with caches is not really relevant to the
> coherency issue.

The relevant part is that if highmem is enabled, copy_user_page() does
not flush the D-cache, leaving it to kunmap_atomic(). Does this latter
function flush the D-cache in all the relevant situations?

-- 
Catalin

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