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Subject: Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte_at()
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <Y9uUO9AadE+8ik/0@arm.com>
Date:   Fri, 3 Feb 2023 10:40:18 +0800
Cc:     Will Deacon <will@kernel.org>, Robin Murphy <robin.murphy@arm.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        linux-arm-kernel@lists.infradead.org,
        Mark Rutland <mark.rutland@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Mark Brown <broonie@kernel.org>
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Message-Id: <F10F1618-7153-41C7-A475-522D833C41D4@linux.dev>
References: <20230109052816.405335-1-anshuman.khandual@arm.com>
 <e924c1aa-5f04-fd7f-52d4-7cf22c476016@arm.com>
 <20230126133321.GB29148@willie-the-truck>
 <d454c9a2-5300-b600-a2ae-21d82d338470@arm.com>
 <20230131154950.GB2646@willie-the-truck> <Y9pZALdn3pKiJUeQ@arm.com>
 <A8DF7D56-C145-4B49-A034-022917B87C89@linux.dev> <Y9uUO9AadE+8ik/0@arm.com>
To:     Catalin Marinas <catalin.marinas@arm.com>
Precedence: bulk


> On Feb 2, 2023, at 18:45, Catalin Marinas <catalin.marinas@arm.com> =
wrote:
>=20
> On Thu, Feb 02, 2023 at 05:51:39PM +0800, Muchun Song wrote:
>>> On Feb 1, 2023, at 20:20, Catalin Marinas <catalin.marinas@arm.com> =
wrote:
>>>> Bah, sorry! Catalin reckons it may have been him talking about the =
vmemmap.
>>>=20
>>> Indeed. The discussion with Anshuman started from this thread:
>>>=20
>>> =
https://lore.kernel.org/all/20221025014215.3466904-1-mawupeng1@huawei.com/=

>>>=20
>>> We already trip over the existing checks even without Anshuman's =
patch,
>>> though only by chance. We are not setting the software PTE_DIRTY on =
the
>>> new pte (we don't bother with this bit for kernel mappings).
>>>=20
>>> Given that the vmemmap ptes are still live when such change happens =
and
>>> no-one came with a solution to the break-before-make problem, I =
propose
>>> we revert the arm64 part of commit 47010c040dec ("mm: =
hugetlb_vmemmap:
>>> cleanup CONFIG_HUGETLB_PAGE_FREE_VMEMMAP*"). We just need this hunk:
>>>=20
>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>> index 27b2592698b0..5263454a5794 100644
>>> --- a/arch/arm64/Kconfig
>>> +++ b/arch/arm64/Kconfig
>>> @@ -100,7 +100,6 @@ config ARM64
>>> select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
>>> select ARCH_WANT_FRAME_POINTERS
>>> select ARCH_WANT_HUGE_PMD_SHARE if ARM64_4K_PAGES || =
(ARM64_16K_PAGES && !ARM64_VA_BITS_36)
>>> - select ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
>>=20
>> Maybe it is a little overkill for HVO as it can significantly =
minimize the
>> overhead of vmemmap on ARM64 servers for some workloads (like qemu, =
DPDK).
>> So I don't think disabling it is a good approach. Indeed, HVO broke =
BBM,
>> but the waring does not affect anything since the tail vmemmap pages =
are
>> supposed to be read-only. So, I suggest skipping warnings if it is =
the
>> vmemmap address in set_pte_at(). What do you think of?
>=20
> IIUC, vmemmap_remap_pte() not only makes the pte read-only but also
> changes the output address. Architecturally, this needs a BBM =
sequence.
> We can avoid going through an invalid pte if we first make the pte
> read-only, TLBI but keeping the same pfn, followed by a change of the
> pfn while keeping the pte readonly. This also assumes that the content
> of the page pointed at by the pte is the same at both old and new pfn.

Right. I think using BBM is to avoid possibly creating multiple TLB =
entries
for the same address for a extremely short period. But accessing either =
the
old page or the new page is fine in this case. Is it acceptable for this
special case without using BBM?

Thanks,
Muchun.