On 25/04/2024 19:58, Zi Yan wrote: > +Anshuman, who changed pmd_present() semantics. See: > https://lore.kernel.org/all/1599627183-14453-2-git-send-email-anshuman.khandual@xxxxxxx/ and commit b65399f6111b ("arm64/mm: Change > THP helpers to comply with generic MM semantics") > > On 25 Apr 2024, at 13:07, 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 > > But pmd_mkinvalid() on x86 does not behave so. Maybe we should fix > pmd_mkinvalid() on arm64 by not setting PMD_PRESENT_INVALID when the > entry is invalid already. And add a test in mm/debug_vm_pgtable.c. Yes, we *could* make pmd_mkinvalid() defensive. But we don't do that for the other getters/setters (e.g. pte_mkwrite()). So not sure why we would want that here. Ultimately it makes no semantic sense to invalidate a non-present pmd. See my other mail for excessive detail. Thanks, Ryan > > I notice that x86, risc-v, mips behave the same. loongarch also > has _PAGE_PRESENT_INVALID bit set during pmd_mkinvalid(), but its > pmd_present() makes sure _PAGE_HUEG is set before checks _PAGE_PRESENT_INVALID. > So it is not a problem for loongarch. Add Huacai to confirm this. > > Maybe pmd_present() on arm64 can do that too?> >> 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. >> I suspect the same is possible on other architectures. >> >> Fix this by only calling pmdp_invalidate() for a present pmd. And for >> good measure let's add a warning to the generic implementation of >> pmdp_invalidate(). I've manually reviewed all other >> pmdp_invalidate[_ad]() call sites and believe all others to be >> conformant. >> >> This is a theoretical bug found during code review. I don't have any >> test case to trigger it in practice. >> >> Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path") >> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx> >> --- >> >> Applies on top of v6.9-rc5. Passes all the mm selftests on arm64. >> >> Thanks, >> Ryan >> >> >> mm/huge_memory.c | 5 +++-- >> mm/pgtable-generic.c | 2 ++ >> 2 files changed, 5 insertions(+), 2 deletions(-) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 89f58c7603b2..80939ad00718 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -2513,12 +2513,12 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> * for this pmd), then we flush the SMP TLB and finally we write the >> * non-huge version of the pmd entry with pmd_populate. >> */ >> - old_pmd = pmdp_invalidate(vma, haddr, pmd); >> >> - pmd_migration = is_pmd_migration_entry(old_pmd); >> + pmd_migration = is_pmd_migration_entry(*pmd); >> if (unlikely(pmd_migration)) { >> swp_entry_t entry; >> >> + old_pmd = *pmd; >> entry = pmd_to_swp_entry(old_pmd); >> page = pfn_swap_entry_to_page(entry); >> write = is_writable_migration_entry(entry); >> @@ -2529,6 +2529,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, >> soft_dirty = pmd_swp_soft_dirty(old_pmd); >> uffd_wp = pmd_swp_uffd_wp(old_pmd); >> } else { >> + old_pmd = pmdp_invalidate(vma, haddr, pmd); >> page = pmd_page(old_pmd); >> folio = page_folio(page); >> if (pmd_dirty(old_pmd)) { >> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c >> index 4fcd959dcc4d..74e34ea90656 100644 >> --- a/mm/pgtable-generic.c >> +++ b/mm/pgtable-generic.c >> @@ -198,6 +198,7 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp) >> pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address, >> pmd_t *pmdp) >> { >> + VM_WARN_ON(!pmd_present(*pmdp)); >> pmd_t old = pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp)); >> flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE); >> return old; >> @@ -208,6 +209,7 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address, >> pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address, >> pmd_t *pmdp) >> { >> + VM_WARN_ON(!pmd_present(*pmdp)); return pmdp_invalidate(vma, address, pmdp); >> } >> #endif >> -- >> 2.25.1 > > > -- > Best Regards, > Yan, Zi
