Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte_at()

[Muchun Song]

> On Feb 3, 2023, at 18:10, Will Deacon <will@xxxxxxxxxx> wrote:
> 
> On Fri, Feb 03, 2023 at 10:40:18AM +0800, Muchun Song wrote:
>> 
>> 
>>> On Feb 2, 2023, at 18:45, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>>> 
>>> On Thu, Feb 02, 2023 at 05:51:39PM +0800, Muchun Song wrote:
>>>>> On Feb 1, 2023, at 20:20, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>>>>>> Bah, sorry! Catalin reckons it may have been him talking about the vmemmap.
>>>>> 
>>>>> Indeed. The discussion with Anshuman started from this thread:
>>>>> 
>>>>> https://lore.kernel.org/all/20221025014215.3466904-1-mawupeng1@xxxxxxxxxx/
>>>>> 
>>>>> 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).
>>>>> 
>>>>> 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:
>>>>> 
>>>>> 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
>>>> 
>>>> 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?
>>> 
>>> 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?
> 
> 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.

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