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

[Muchun Song <muchun.song@linux.dev>]

> On Feb 2, 2023, at 18:45, Catalin Marinas <catalin.marinas@arm.com> 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@arm.com> 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@huawei.com/
>>> 
>>> 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?

Thanks,
Muchun.