On Feb 1, 2023, at 20:20, Catalin Marinas [off-list ref] wrote:

On Tue, Jan 31, 2023 at 03:49:51PM +0000, Will Deacon wrote:

quoted

On Fri, Jan 27, 2023 at 12:43:17PM +0000, Robin Murphy wrote:

quoted

On 2023-01-26 13:33, Will Deacon wrote:

quoted

On Tue, Jan 24, 2023 at 11:11:49AM +0530, Anshuman Khandual wrote:

quoted

On 1/9/23 10:58, Anshuman Khandual wrote:

quoted

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 <redacted>
---
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/ (local)

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