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

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Message-ID: <20100305012823.GD26618@linux-sh.org>
Date:	Fri, 5 Mar 2010 10:28:23 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	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, Mar 04, 2010 at 09:44:35PM +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.
> 
> I wonder if there's any other anonymous page creating functions which
> could do with a similar treatment...

In the anonymous page case, flush_anon_page() is always called prior to
flush_dcache_page(), so flush_anon_page() could just set PG_dcache_clean
to work around that. That would handle get_user_pages(), too.
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