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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: <20230203101023.GA5597@willie-the-truck>
Date:   Mon, 6 Feb 2023 11:28:12 +0800
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        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>
Content-Transfer-Encoding: quoted-printable
Message-Id: <93461768-791A-45BE-BEF2-344CC5228C92@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>
 <F10F1618-7153-41C7-A475-522D833C41D4@linux.dev>
 <20230203101023.GA5597@willie-the-truck>
To:     Will Deacon <will@kernel.org>
Precedence: bulk


> On Feb 3, 2023, at 18:10, Will Deacon <will@kernel.org> wrote:
>=20
> On Fri, Feb 03, 2023 at 10:40:18AM +0800, Muchun Song wrote:
>>=20
>>=20
>>> 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.
>>=20
>> 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?
>=20
> Sadly, the architecture allows the CPU to conjure up a mapping based =
on a
> combination of the old and the new descriptor (a process known as
> "amalgamation") so we _really_ need the BBM sequence.

I am not familiar with ARM64, what's the user-visible effect if this
"amalgamation" occurs?

Muchun,
Thanks.

>=20
> I'm in favour of disabling the optimisation now and bringing it back =
once
> we've got this fixed.
>=20
> Will