[PATCH 08/10] mm: update fork mm->flags initialisation to use bitmap

Lorenzo Stoakes
2025-08-12T15:48:38Z

We now need to account for flag initialisation on fork. We retain the
existing logic as much as we can, but dub the existing flag mask legacy.

These flags are therefore required to fit in the first 32-bits of the flags
field.

However, further flag propagation upon fork can be implemented in mm_init()
on a per-flag basis.

We ensure we clear the entire bitmap prior to setting it, and use
__mm_flags_get_word() and __mm_flags_set_word() to manipulate these legacy
fields efficiently.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
include/linux/mm_types.h | 13 ++++++++++---
kernel/fork.c | 7 +++++--
2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 38b3fa927997..25577ab39094 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1820,16 +1820,23 @@ enum {
#define MMF_TOPDOWN 31 /* mm searches top down by default */
#define MMF_TOPDOWN_MASK _BITUL(MMF_TOPDOWN)

-#define MMF_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
+#define MMF_INIT_LEGACY_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\
MMF_VM_MERGE_ANY_MASK | MMF_TOPDOWN_MASK)

-static inline unsigned long mmf_init_flags(unsigned long flags)
+/* Legacy flags must fit within 32 bits. */
+static_assert((u64)MMF_INIT_LEGACY_MASK <= (u64)UINT_MAX);
+
+/*
+ * Initialise legacy flags according to masks, propagating selected flags on
+ * fork. Further flag manipulation can be performed by the caller.
+ */
+static inline unsigned long mmf_init_legacy_flags(unsigned long flags)
{
if (flags & (1UL << MMF_HAS_MDWE_NO_INHERIT))
flags &= ~((1UL << MMF_HAS_MDWE) |
(1UL << MMF_HAS_MDWE_NO_INHERIT));
- return flags & MMF_INIT_MASK;
+ return flags & MMF_INIT_LEGACY_MASK;
}

#endif /* _LINUX_MM_TYPES_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index c4ada32598bd..b311caec6419 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1056,11 +1056,14 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
mm_init_uprobes_state(mm);
hugetlb_count_init(mm);

+ mm_flags_clear_all(mm);
if (current->mm) {
- mm->flags = mmf_init_flags(current->mm->flags);
+ unsigned long flags = __mm_flags_get_word(current->mm);
+
+ __mm_flags_set_word(mm, mmf_init_legacy_flags(flags));
mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
} else {
- mm->flags = default_dump_filter;
+ __mm_flags_set_word(mm, default_dump_filter);
mm->def_flags = 0;
}

--
2.50.1