linux-kernel - Re: [PATCH 0/1] Fixup write permission of TLB on powerpc e500 core

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Date:	Fri, 15 Jul 2011 19:07:15 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	MailingLists <haishan.bai@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, paulus@...ba.org,
	tglx@...utronix.de, walken@...gle.com, dhowells@...hat.com,
	cmetcalf@...era.com, tony.luck@...el.com,
	akpm@...ux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] Fixup write permission of TLB on powerpc e500 core

On Fri, 2011-07-15 at 16:38 +0800, MailingLists wrote:
> A page could be set to read only by the kernel (supervisor in the
> powerpc
> literature) on the e500, and that's what the kernel do. Set
> SW(supervisor
> write) bit in the TLB entry to grant write permission to the kernel on
> a
> page.
> 
> And further the SW bit is set according to the DIRTY flag of the PTE,
> PTE.DIRTY is set in the do_page_fault(), the futex_lock_pi() disabled
> page fault, the PTE.DIRTY never can be set, so do the SW bit,
> unbreakable
> COW occurred, infinite loop followed. 

That would be it ... the SW dirty and young tracking relies on faults to
fixup things in handle_pte_fault(). If the "disable page fault" thingy
happens before we get there, then we have a pretty nasty bug. Note that
this will hit more than just e500 (and in fact any architecture that
relies on SW to do dirty and young tracking).

Cheers,
Ben.

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