On 01.05.24 16:33, 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. This is a problem for the migration entry case because pmd_mkinvalid(), called by pmdp_invalidate() must only be called for a present pmd. On arm64 at least, pmd_mkinvalid() will mark the pmd such that any future call to pmd_present() will return true. And therefore any lockless pgtable walker could see the migration entry pmd in this state and start interpretting the fields as if it were present, leading to BadThings (TM). GUP-fast appears to be one such lockless pgtable walker. x86 does not suffer the above problem, but instead pmd_mkinvalid() will corrupt the offset field of the swap entry within the swap pte. See link below for discussion of that problem.
Could that explain: https://lore.kernel.org/all/YjoGbhreg8lGCGIJ@xxxxxxxxxxxxx/ Where the PFN of a migration entry might have been corrupted? Ccing Felix Patch itself looks good to me Acked-by: David Hildenbrand <david@xxxxxxxxxx> -- Cheers, David / dhildenb
