linux-kernel - Re: [PATCH] [ARM] force dcache flush if dcache

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Date:	Wed, 14 Oct 2009 18:10:29 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	David Miller <davem@...emloft.net>,
	Nitin Gupta <ngupta@...are.org>, Nick Piggin <npiggin@...e.de>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH] [ARM] force dcache flush if dcache_dirty bit set

On Tue, 2009-10-13 at 11:07 +0900, Paul Mundt wrote:
> On Mon, Oct 12, 2009 at 06:03:12PM +0100, Russell King wrote:
> > On Mon, Oct 12, 2009 at 05:09:53PM +0100, Hugh Dickins wrote:
> > > Sorry to muddy the waters on this, if you and Dave are sure that
> > > you have the right fix, down in your architectures, and that fix
> > > isn't going to hurt your performance significantly.
> > 
> > If I look at the issue from this point of view:
> > 
> > - we are using PG_arch_1 to delay cache handling for the page
> > 
> > - if PG_arch_1 is set on a page, we set it explicitly because we
> >   didn't do some flushing between the allocation of the page and
> >   mapping it into userspace
> > 
> > - if a page with PG_arch_1 set ever gets to userspace, this can
> >   only be because we did the lazy flushing thing
> > 
> > I don't see that there should have been any bearing on whether a page
> > has a mapping or not when we get to update_mmu_cache.  The issue here
> > is that > if PG_arch_1 is set on a page, then we didn't flush it at
> > the time when we believed it was appropriate to do so. <
> > 
> > Tell me I'm wrong (having only just sent it to Linus...)
> 
> Having looked at the ARM fix, in the !mapping case do you not need the
> I-cache flush on vma->vm_flags & VM_EXEC? Or is the presumption that
> flush_icache_page()-type action doesn't need to be undertaken by
> flush_dcache_page()/update_mmu_cache() when there is no page_mapping()?

If I understand Nitin's scenario correctly, I think it should also
invalidate the I-cache.

For executable anonymous pages containing, it's the user app writing the
code (JIT etc.) and it calls an ARM-specific syscall for I and D cache
maintenance. If such page is read back from swap, following Nitin's
scenario, the I-cache would need to be invalidated as well otherwise it
can have stale entries.

-- 
Catalin

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