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

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Date:	Fri, 05 Mar 2010 16:52:40 +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 Thu, 2010-03-04 at 21:44 +0000, Russell King - ARM Linux wrote:
> On Thu, Mar 04, 2010 at 06:36:42PM +0000, Catalin Marinas wrote:
> > ARM: Assume new page cache pages have dirty D-cache
> >
> > From: Catalin Marinas <catalin.marinas@....com>
> >
> > There are places in Linux where writes to newly allocated page cache
> > pages happen without a subsequent call to flush_dcache_page() (several
> > PIO drivers including USB HCD). This patch changes the meaning of
> > PG_arch_1 to be PG_dcache_clean and always flush the D-cache for a newly
> > mapped page in update_mmu_cache().
> >
> > The patch also sets the PG_arch_1 bit in the DMA cache maintenance
> > function to avoid additional cache flushing in update_mmu_cache().
> 
> As I just realised, this is going to subject all pages placed into
> userspace with a D cache flush - even anonymous pages, and those
> which we've been careful to deal with the cache issues already (eg,
> via the COW page copying code.)
> 
> I think all the copypage functions need to set PG_dcache_clean on the
> new pages once their copy has completed.

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.

But for non-aliasing VIPT, do we actually need the D-cache flushing in
kunmap_atomic() or copy_user_page()? If we default to new page being
dirty, I think we can remove the cache cleaning from this function and
leave it for update_mmu_cache (or set_pte_at but that's for a different
patch).

-- 
Catalin

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