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From: Catalin Marinas <catalin.marinas@arm.com>
To: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org, kernel-team@android.com,
	Yu Zhao <yuzhao@google.com>, Minchan Kim <minchan@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
	stable@vger.kernel.org
Subject: Re: [PATCH 2/6] arm64: pgtable: Ensure dirty bit is preserved across pte_wrprotect()
Date: Mon, 23 Nov 2020 14:22:37 +0000	[thread overview]
Message-ID: <20201123142237.GF17833@gaia> (raw)
In-Reply-To: <20201120143557.6715-3-will@kernel.org>

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.

My assumption at the time was that the caller of pte_wrprotect() already
moved the 'dirty' information to the underlying page. Most
pte_wrprotect() calls also do a pte_mkclean(). However, it doesn't seem
to always be the case (soft-dirty but we don't support it yet).

I was worried that we may inadvertently set the dirty bit when doing a
pte_wrprotect() on a freshly created pte (not read from memory, for
example __split_huge_pmd_locked()) but I think all our __P* and __S*
attributes start with a PTE_RDONLY, therefore the pte_hw_dirty() returns
false. A test for mm/debug_vm_pgtable.c, something like:

	for (i = 0, i < ARRAY_SIZE(protection_map); i++) {
		pte = pfn_pte(pfn, protection_map(i));
		WARN_ON(pte_dirty(pte_wrprotect(pte));
	}

(I'll leave this to Anshuman ;))

> 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>
> Signed-off-by: Will Deacon <will@kernel.org>

I think this could go back as far as the hardware AF/DBM support (v4.3):

Fixes: 2f4b829c625e ("arm64: Add support for hardware updates of the access and dirty pte bits")

If you limit this fix to 4.14, you probably don't need additional
commits. Otherwise, at least this one:

3bbf7157ac66 ("arm64: Convert pte handling from inline asm to using (cmp)xchg")

and a slightly more intrusive:

73e86cb03cf2 ("arm64: Move PTE_RDONLY bit handling out of set_pte_at()")

We also had some attempts at fixing ptep_set_wrprotect():

64c26841b349 ("arm64: Ignore hardware dirty bit updates in ptep_set_wrprotect()")

Fixed subsequently by:

8781bcbc5e69 ("arm64: mm: Fix pte_mkclean, pte_mkdirty semantics")

I have a hope that at some point we'll understand how this all works ;).

For this patch:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

      parent reply	other threads:[~2020-11-23 14:22 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
2020-11-23 14:22   ` Catalin Marinas [this message]

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