Re: [PATCH v9 12/17] mm: move lesser used vma_area_struct members into the last cacheline

Lorenzo Stoakes
2025-01-13T16:16:37Z

On Fri, Jan 10, 2025 at 08:25:59PM -0800, Suren Baghdasaryan wrote:
> Move several vma_area_struct members which are rarely or never used
> during page fault handling into the last cacheline to better pack
> vm_area_struct. As a result vm_area_struct will fit into 3 as opposed
> to 4 cachelines. New typical vm_area_struct layout:
>
> struct vm_area_struct {
> union {
> struct {
> long unsigned int vm_start; /* 0 8 */
> long unsigned int vm_end; /* 8 8 */
> }; /* 0 16 */
> freeptr_t vm_freeptr; /* 0 8 */
> }; /* 0 16 */
> struct mm_struct * vm_mm; /* 16 8 */
> pgprot_t vm_page_prot; /* 24 8 */
> union {
> const vm_flags_t vm_flags; /* 32 8 */
> vm_flags_t __vm_flags; /* 32 8 */
> }; /* 32 8 */
> unsigned int vm_lock_seq; /* 40 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> struct list_head anon_vma_chain; /* 48 16 */
> /* --- cacheline 1 boundary (64 bytes) --- */
> struct anon_vma * anon_vma; /* 64 8 */
> const struct vm_operations_struct * vm_ops; /* 72 8 */
> long unsigned int vm_pgoff; /* 80 8 */
> struct file * vm_file; /* 88 8 */
> void * vm_private_data; /* 96 8 */
> atomic_long_t swap_readahead_info; /* 104 8 */
> struct mempolicy * vm_policy; /* 112 8 */
> struct vma_numab_state * numab_state; /* 120 8 */
> /* --- cacheline 2 boundary (128 bytes) --- */
> refcount_t vm_refcnt (__aligned__(64)); /* 128 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> struct {
> struct rb_node rb (__aligned__(8)); /* 136 24 */
> long unsigned int rb_subtree_last; /* 160 8 */
> } __attribute__((__aligned__(8))) shared; /* 136 32 */
> struct anon_vma_name * anon_name; /* 168 8 */
> struct vm_userfaultfd_ctx vm_userfaultfd_ctx; /* 176 8 */
>
> /* size: 192, cachelines: 3, members: 18 */
> /* sum members: 176, holes: 2, sum holes: 8 */
> /* padding: 8 */
> /* forced alignments: 2, forced holes: 1, sum forced holes: 4 */
> } __attribute__((__aligned__(64)));
>
> Memory consumption per 1000 VMAs becomes 48 pages:
>
> slabinfo after vm_area_struct changes:
> <name> ... <objsize> <objperslab> <pagesperslab> : ...
> vm_area_struct ... 192 42 2 : ...
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

Looks sensible to me:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
> include/linux/mm_types.h | 38 ++++++++++++++++++--------------------
> 1 file changed, 18 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 9228d19662c6..d902e6730654 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -725,17 +725,6 @@ struct vm_area_struct {
> */
> unsigned int vm_lock_seq;
> #endif
> -
> - /*
> - * For areas with an address space and backing store,
> - * linkage into the address_space->i_mmap interval tree.
> - *
> - */
> - struct {
> - struct rb_node rb;
> - unsigned long rb_subtree_last;
> - } shared;
> -
> /*
> * A file's MAP_PRIVATE vma can be in both i_mmap tree and anon_vma
> * list, after a COW of one of the file pages. A MAP_SHARED vma
> @@ -755,14 +744,6 @@ struct vm_area_struct {
> struct file * vm_file; /* File we map to (can be NULL). */
> void * vm_private_data; /* was vm_pte (shared mem) */
>
> -#ifdef CONFIG_ANON_VMA_NAME
> - /*
> - * For private and shared anonymous mappings, a pointer to a null
> - * terminated string containing the name given to the vma, or NULL if
> - * unnamed. Serialized by mmap_lock. Use anon_vma_name to access.
> - */
> - struct anon_vma_name *anon_name;
> -#endif
> #ifdef CONFIG_SWAP
> atomic_long_t swap_readahead_info;
> #endif
> @@ -775,7 +756,6 @@ struct vm_area_struct {
> #ifdef CONFIG_NUMA_BALANCING
> struct vma_numab_state *numab_state; /* NUMA Balancing state */
> #endif
> - struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> #ifdef CONFIG_PER_VMA_LOCK
> /* Unstable RCU readers are allowed to read this. */
> refcount_t vm_refcnt ____cacheline_aligned_in_smp;
> @@ -783,6 +763,24 @@ struct vm_area_struct {
> struct lockdep_map vmlock_dep_map;
> #endif
> #endif
> + /*
> + * For areas with an address space and backing store,
> + * linkage into the address_space->i_mmap interval tree.
> + *
> + */
> + struct {
> + struct rb_node rb;
> + unsigned long rb_subtree_last;
> + } shared;
> +#ifdef CONFIG_ANON_VMA_NAME
> + /*
> + * For private and shared anonymous mappings, a pointer to a null
> + * terminated string containing the name given to the vma, or NULL if
> + * unnamed. Serialized by mmap_lock. Use anon_vma_name to access.
> + */
> + struct anon_vma_name *anon_name;
> +#endif
> + struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> } __randomize_layout;
>
> #ifdef CONFIG_NUMA
> --
> 2.47.1.613.gc27f4b7a9f-goog
>