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

[Liam R. Howlett]

* Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [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@xxxxxxxxxx>

Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

> ---
>  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
>