On Tue, Apr 30, 2024 at 06:57:52PM +0100, Catalin Marinas wrote:
On Tue, 30 Apr 2024 14:31:38 +0100, Ryan Roberts wrote:
__split_huge_pmd_locked() can be called for a present THP, devmap or (non-present) migration entry. It calls pmdp_invalidate() unconditionally on the pmdp and only determines if it is present or not based on the returned old pmd.
But arm64's pmd_mkinvalid(), called by pmdp_invalidate(), unconditionally sets the PMD_PRESENT_INVALID flag, which causes future pmd_present() calls to return true - even for a swap pmd. Therefore any lockless pgtable walker could see the migration entry pmd in this state and start interpretting the fields (e.g. pmd_pfn()) as if it were present, leading to BadThings (TM). GUP-fast appears to be one such lockless pgtable walker.
[...]
Applied to arm64 (for-next/fixes), thanks! It should land in 6.9-rc7. I removed the debug/test code, please send it as a separate patch for 6.10.
[1/1] arm64/mm: pmd_mkinvalid() must handle swap pmds https://git.kernel.org/arm64/c/e783331c7720
Since Andrew merged the generic mm fix, I dropped this patch from the arm64 for-next/fixes branch.