that is registered for UFFD in MINOR mode, it accidentally clears all flags stored in the upper 32 bits of vma->vm_flags. Assuming x86_64 kernel build, unsigned long is 64-bit and unsigned int and int are 32-bit wide. This setup causes the following mishap during the &= ~VM_MERGEABLE assignment. VM_MERGEABLE is a 32-bit constant of type unsigned int, 0x8000'0000. After ~ is applied, it becomes 0x7fff'ffff unsigned int, which is then promoted to unsigned long before the & operation. This promotion fills upper 32 bits with leading 0s, as we're doing unsigned conversion (and even for a signed conversion, this wouldn't help as the leading bit is 0). & operation thus ends up AND-ing vm_flags with 0x0000'0000'7fff'ffff instead of intended 0xffff'ffff'7fff'ffff and hence accidentally clears the upper 32-bits of its value. Fix it by changing `VM_MERGEABLE` constant to unsigned long, using the BIT() macro. Note: other VM_* flags are not affected: This only happens to the VM_MERGEABLE flag, as the other VM_* flags are all constants of type int and after ~ operation, they end up with leading 1 and are thus converted to unsigned long with leading 1s. Note 2: After commit 31defc3b01d9 ("userfaultfd: remove (VM_)BUG_ON()s"), this is no longer a kernel BUG, but a WARNING at the same place: [ 45.595973] WARNING: CPU: 1 PID: 2474 at mm/userfaultfd.c:2067 but the root-cause (flag-drop) remains the same. [akpm@linux-foundation.org: rust bindgen wasn't able to handle BIT(), from Miguel] Link: https://lore.kernel.org/oe-kbuild-all/202510030449.VfSaAjvd-lkp@intel.com/ Link: https://lkml.kernel.org/r/20251001090353.57523-2-acsjakub@amazon.de Fixes: 7677f7fd8be7 ("userfaultfd: add minor fault registration mode") Signed-off-by: Jakub Acs Signed-off-by: Miguel Ojeda Acked-by: David Hildenbrand Acked-by: SeongJae Park Tested-by: Alice Ryhl Tested-by: Miguel Ojeda Cc: Xu Xin Cc: Chengming Zhou Cc: Peter Xu Cc: Axel Rasmussen Cc: Signed-off-by: Andrew Morton --- include/linux/mm.h | 2 +- rust/bindings/bindings_helper.h | 1 + 2 files changed, 2 insertions(+), 1 deletion(-) --- a/include/linux/mm.h~mm-ksm-fix-flag-dropping-behavior-in-ksm_madvise +++ a/include/linux/mm.h @@ -323,7 +323,7 @@ extern unsigned int kobjsize(const void #define VM_MIXEDMAP 0x10000000 /* Can contain "struct page" and pure PFN pages */ #define VM_HUGEPAGE 0x20000000 /* MADV_HUGEPAGE marked this vma */ #define VM_NOHUGEPAGE 0x40000000 /* MADV_NOHUGEPAGE marked this vma */ -#define VM_MERGEABLE 0x80000000 /* KSM may merge identical pages */ +#define VM_MERGEABLE BIT(31) /* KSM may merge identical pages */ #ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS #define VM_HIGH_ARCH_BIT_0 32 /* bit only usable on 64-bit architectures */ --- a/rust/bindings/bindings_helper.h~mm-ksm-fix-flag-dropping-behavior-in-ksm_madvise +++ a/rust/bindings/bindings_helper.h @@ -108,6 +108,7 @@ const xa_mark_t RUST_CONST_HELPER_XA_PRE const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC = XA_FLAGS_ALLOC; const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC1 = XA_FLAGS_ALLOC1; +const vm_flags_t RUST_CONST_HELPER_VM_MERGEABLE = VM_MERGEABLE; #if IS_ENABLED(CONFIG_ANDROID_BINDER_IPC_RUST) #include "../../drivers/android/binder/rust_binder.h" _ Patches currently in -mm which might be from acsjakub@amazon.de are mm-redefine-vm_-flag-constants-with-bit.patch[merged mm-hotfixes-stable] mm-ksm-fix-flag-dropping-behavior-in-ksm_madvise.patch removed from -mm treeAndrew Morton undefinedmm-commits@vger.kernel.org,xu.xin16@zte.com.cn,stable@vger.kernel.org,sj@kernel.org,peterx@redhat.com,miguel.ojeda.sandonis@gmail.com,david@redhat.com,chengming.zhou@linux.dev,axelrasmussen@google.com,aliceryhl@google.com,acsjakub@amazon.de,akpm@linux-foundation.org undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined—@ƒ†[