On Wed, May 01, 2024 at 09:05:17AM +0100, Ryan Roberts wrote: > On 30/04/2024 18:57, 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. > > Thanks Catalin! I'm guessing this will turn up in today's linux-next, so if I > send the tests today and Andrew puts them straight in mm-unstable (which will > goto linux-next) there is no risk that the tests are there without the fix? Or > do I need to hold off until the fix is in v6.9-rc7? It looks like we don't push for-next/fixes to linux-next, it's short-lived usually, it ends up upstream quickly. I can send the pull request later today, should turn up in mainline by tomorrow. You can add a note to your patch for Andrew that it will fail on arm64 until the fix ends up upstream. It's a matter of a couple of days anyway. -- Catalin
