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From: Catalin Marinas <catalin.marinas@arm.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Will Deacon <will@kernel.org>,
	kernel-team@android.com, Yu Zhao <yuzhao@google.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	linux-mm@kvack.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/6] arm64: pgtable: Ensure dirty bit is preserved across pte_wrprotect()
Date: Mon, 23 Nov 2020 14:31:49 +0000	[thread overview]
Message-ID: <20201123143149.GG17833@gaia> (raw)
In-Reply-To: <20201120170903.GC3377168@google.com>

On Fri, Nov 20, 2020 at 09:09:03AM -0800, Minchan Kim wrote:
> On Fri, Nov 20, 2020 at 02:35:53PM +0000, Will Deacon wrote:
> > With hardware dirty bit management, calling pte_wrprotect() on a writable,
> > dirty PTE will lose the dirty state and return a read-only, clean entry.
> > 
> > Move the logic from ptep_set_wrprotect() into pte_wrprotect() to ensure that
> > the dirty bit is preserved for writable entries, as this is required for
> > soft-dirty bit management if we enable it in the future.
> > 
> > Cc: <stable@vger.kernel.org>
> 
> It this stable material if it would be a problem once ARM64 supports
> softdirty in future?

I don't think so. Arm64 did not have a hardware dirty mechanism from the
start, it was added later but in a way as to coexist with other CPUs or
peripherals that don't support it. So instead of setting a PTE_DIRTY bit
as one would expect, the CPU clears the PTE_RDONLY on write access to a
writable PTE (the PTE_DBM/PTE_WRITE bit set). So our pte_wrprotect()
needs to set PTE_RDONLY and clear PTE_DBM (PTE_WRITE) but !PTE_RDONLY is
our only information of a pte having been dirtied, so we have to
transfer it to a software PTE_DIRTY bit. This is different from a
soft-dirty pte bit if we add it in the future.

-- 
Catalin

  reply	other threads:[~2020-11-23 14:31 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20201120143557.6715-1-will@kernel.org>
2020-11-20 14:35 ` [PATCH 1/6] arm64: pgtable: Fix pte_accessible() Will Deacon
2020-11-20 16:03   ` Minchan Kim
2020-11-20 19:53   ` Yu Zhao
2020-11-23 13:27   ` Catalin Marinas
2020-11-24 10:02   ` Anshuman Khandual
2020-11-20 14:35 ` [PATCH 2/6] arm64: pgtable: Ensure dirty bit is preserved across pte_wrprotect() Will Deacon
2020-11-20 17:09   ` Minchan Kim
2020-11-23 14:31     ` Catalin Marinas [this message]
2020-11-23 14:22   ` Catalin Marinas

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