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

[Will Deacon <will@kernel.org>]

On Mon, Feb 06, 2023 at 11:28:12AM +0800, Muchun Song wrote:
> 
> 
> > On Feb 3, 2023, at 18:10, Will Deacon <will@kernel.org> 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@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?
> > 
> > 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?

The user-visible effects would probably be data corruption and instability,
since the amalgamated TLB entry could result in a bogus physical address and
bogus permissions.

Will

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