Re: [PATCH 01/10] mm: add bitmap mm->flags field

Liam R. Howlett
2025-08-12T16:21:19Z

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [250812 11:47]:
> We are currently in the bizarre situation where we are constrained on the
> number of flags we can set in an mm_struct based on whether this is a
> 32-bit or 64-bit kernel.
>
> This is because mm->flags is an unsigned long field, which is 32-bits on a
> 32-bit system and 64-bits on a 64-bit system.
>
> In order to keep things functional across both architectures, we do not
> permit mm flag bits to be set above flag 31 (i.e. the 32nd bit).
>
> This is a silly situation, especially given how profligate we are in
> storing metadata in mm_struct, so let's convert mm->flags into a bitmap and
> allow ourselves as many bits as we like.
>
> To keep things manageable, firstly we introduce the bitmap at a system word
> system as a new field mm->_flags, in union.
>
> This means the new bitmap mm->_flags is bitwise exactly identical to the
> existing mm->flags field.
>
> We have an opportunity to also introduce some type safety here, so let's
> wrap the mm flags field as a struct and declare it as an mm_flags_t typedef
> to keep it consistent with vm_flags_t for VMAs.
>
> We make the internal field privately accessible, in order to force the use
> of helper functions so we can enforce that accesses are bitwise as
> required.
>
> We therefore introduce accessors prefixed with mm_flags_*() for callers to
> use. We place the bit parameter first so as to match the parameter ordering
> of the *_bit() functions.
>
> Having this temporary union arrangement allows us to incrementally swap
> over users of mm->flags patch-by-patch rather than having to do everything
> in one fell swoop.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
> include/linux/mm.h | 32 ++++++++++++++++++++++++++++++++
> include/linux/mm_types.h | 39 ++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 70 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 3868ca1a25f9..4ed4a0b9dad6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -34,6 +34,8 @@
> #include <linux/slab.h>
> #include <linux/cacheinfo.h>
> #include <linux/rcuwait.h>
> +#include <linux/bitmap.h>
> +#include <linux/bitops.h>
>
> struct mempolicy;
> struct anon_vma;
> @@ -720,6 +722,36 @@ static inline void assert_fault_locked(struct vm_fault *vmf)
> }
> #endif /* CONFIG_PER_VMA_LOCK */
>
> +static inline bool mm_flags_test(int flag, const struct mm_struct *mm)
> +{
> + return test_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline bool mm_flags_test_and_set(int flag, struct mm_struct *mm)
> +{
> + return test_and_set_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline bool mm_flags_test_and_clear(int flag, struct mm_struct *mm)
> +{
> + return test_and_clear_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline void mm_flags_set(int flag, struct mm_struct *mm)
> +{
> + set_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline void mm_flags_clear(int flag, struct mm_struct *mm)
> +{
> + clear_bit(flag, ACCESS_PRIVATE(&mm->_flags, __mm_flags));
> +}
> +
> +static inline void mm_flags_clear_all(struct mm_struct *mm)
> +{
> + bitmap_zero(ACCESS_PRIVATE(&mm->_flags, __mm_flags), NUM_MM_FLAG_BITS);
> +}
> +
> extern const struct vm_operations_struct vma_dummy_vm_ops;
>
> static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index cf94df4955c7..46d3fb8935c7 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -20,6 +20,7 @@
> #include <linux/seqlock.h>
> #include <linux/percpu_counter.h>
> #include <linux/types.h>
> +#include <linux/bitmap.h>
>
> #include <asm/mmu.h>
>
> @@ -927,6 +928,15 @@ struct mm_cid {
> };
> #endif
>
> +/*
> + * Opaque type representing current mm_struct flag state. Must be accessed via
> + * mm_flags_xxx() helper functions.
> + */
> +#define NUM_MM_FLAG_BITS BITS_PER_LONG
> +typedef struct {
> + __private DECLARE_BITMAP(__mm_flags, NUM_MM_FLAG_BITS);
> +} mm_flags_t;
> +
> struct kioctx_table;
> struct iommu_mm_data;
> struct mm_struct {
> @@ -1109,7 +1119,11 @@ struct mm_struct {
> /* Architecture-specific MM context */
> mm_context_t context;
>
> - unsigned long flags; /* Must use atomic bitops to access */
> + /* Temporary union while we convert users to mm_flags_t. */
> + union {
> + unsigned long flags; /* Must use atomic bitops to access */
> + mm_flags_t _flags; /* Must use mm_flags_* helpers to access */
> + };
>
> #ifdef CONFIG_AIO
> spinlock_t ioctx_lock;
> @@ -1219,6 +1233,29 @@ struct mm_struct {
> unsigned long cpu_bitmap[];
> };
>
> +/* Read the first system word of mm flags, non-atomically. */
> +static inline unsigned long __mm_flags_get_word(struct mm_struct *mm)
> +{
> + unsigned long *bitmap = ACCESS_PRIVATE(&mm->_flags, __mm_flags);
> +
> + return bitmap_read(bitmap, 0, BITS_PER_LONG);
> +}
> +
> +/* Set the first system word of mm flags, non-atomically. */
> +static inline void __mm_flags_set_word(struct mm_struct *mm,
> + unsigned long value)
> +{
> + unsigned long *bitmap = ACCESS_PRIVATE(&mm->_flags, __mm_flags);
> +
> + bitmap_copy(bitmap, &value, BITS_PER_LONG);
> +}
> +
> +/* Obtain a read-only view of the bitmap. */
> +static inline const unsigned long *__mm_flags_get_bitmap(const struct mm_struct *mm)
> +{
> + return (const unsigned long *)ACCESS_PRIVATE(&mm->_flags, __mm_flags);
> +}
> +
> #define MM_MT_FLAGS (MT_FLAGS_ALLOC_RANGE | MT_FLAGS_LOCK_EXTERN | \
> MT_FLAGS_USE_RCU)
> extern struct mm_struct init_mm;
> --
> 2.50.1
>