Messages in this thread Patch in this message | | | Date | Wed, 1 Feb 2023 12:20:16 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte_at() |
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On Tue, Jan 31, 2023 at 03:49:51PM +0000, Will Deacon wrote: > On Fri, Jan 27, 2023 at 12:43:17PM +0000, Robin Murphy wrote: > > On 2023-01-26 13:33, Will Deacon wrote: > > > On Tue, Jan 24, 2023 at 11:11:49AM +0530, Anshuman Khandual wrote: > > > > On 1/9/23 10:58, Anshuman Khandual wrote: > > > > > Changing pfn on a user page table mapped entry, without first going through > > > > > break-before-make (BBM) procedure is unsafe. This just updates set_pte_at() > > > > > to intercept such changes, via an updated pgattr_change_is_safe(). This new > > > > > check happens via __check_racy_pte_update(), which has now been renamed as > > > > > __check_safe_pte_update(). > > > > > > > > > > Cc: Catalin Marinas <catalin.marinas@arm.com> > > > > > Cc: Will Deacon <will@kernel.org> > > > > > Cc: Mark Rutland <mark.rutland@arm.com> > > > > > Cc: Andrew Morton <akpm@linux-foundation.org> > > > > > Cc: linux-arm-kernel@lists.infradead.org > > > > > Cc: linux-kernel@vger.kernel.org > > > > > Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> > > > > > --- > > > > > This applies on v6.2-rc3. This patch had some test time on an internal CI > > > > > system without any issues being reported. > > > > > > > > Gentle ping, any updates on this patch ? Still any concerns ? > > > > > > I don't think we really got to the bottom of Mark's concerns with > > > unreachable ptes on the stack, did we? I also have vague recollections > > > of somebody (Robin?) running into issues with the vmap code not honouring > > > BBM. > > > > Doesn't ring a bell, so either it wasn't me, or it was many years ago and > > about 5 levels deep into trying to fix something else :/ > > 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 select ARCH_WANT_LD_ORPHAN_WARN select ARCH_WANTS_NO_INSTR select ARCH_WANTS_THP_SWAP if ARM64_4K_PAGES -- Catalin
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