Re: [PATCH v2 0/5] mm/vmalloc.c: improve readability and rewrite vmap_area

[Pengfei Li <lpf.vector@gmail.com>]

On Thu, Jul 4, 2019 at 3:30 AM Uladzislau Rezki <urezki@gmail.com> wrote:
>
> Hello, Li.
>
> I do not think that it is worth to reduce the struct size the way
> this series does. I mean the union around flags/va_start. Simply saying
> if we need two variables: flags and va_start let's have them. Otherwise
> everybody has to think what he/she access at certain moment of time.
>
> So it would be easier to make mistakes, also that conversion looks strange
> to me. That is IMHO.
>
> If we want to reduce the size to L1-cache-line(64 bytes), i would propose to
> eliminate the "flags" variable from the structure. We could do that if apply
> below patch(as an example) on top of https://lkml.org/lkml/2019/7/3/661:
>

Hi, Vlad

Thank you for your detailed comments!

What you said inspired me. I really have no reason to stubbornly
keep the "flags" in vmap_area since it can be eliminated.

I will eliminate the "flags" from vmap_area as you suggested, and
the next version will be based on top of your commit
https://lkml.org/lkml/2019/7/3/661.


-- 
Pengfei

> <snip>
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 51e131245379..49bb82863d5b 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -51,15 +51,22 @@ struct vmap_area {
>         unsigned long va_start;
>         unsigned long va_end;
>
> -       /*
> -        * Largest available free size in subtree.
> -        */
> -       unsigned long subtree_max_size;
> -       unsigned long flags;
>         struct rb_node rb_node;         /* address sorted rbtree */
>         struct list_head list;          /* address sorted list */
> -       struct llist_node purge_list;    /* "lazy purge" list */
> -       struct vm_struct *vm;
> +
> +       /*
> +        * Below three variables can be packed, because vmap_area
> +        * object can be only in one of the three different states:
> +        *
> +        * - when an object is in "free" tree only;
> +        * - when an object is in "purge list" only;
> +        * - when an object is in "busy" tree only.
> +        */
> +       union {
> +               unsigned long subtree_max_size;
> +               struct llist_node purge_list;
> +               struct vm_struct *vm;
> +       };
>  };
>
>  /*
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 6f1b6a188227..e389a6db222b 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -329,8 +329,6 @@ EXPORT_SYMBOL(vmalloc_to_pfn);
>  #define DEBUG_AUGMENT_PROPAGATE_CHECK 0
>  #define DEBUG_AUGMENT_LOWEST_MATCH_CHECK 0
>
> -#define VM_VM_AREA     0x04
> -
>  static DEFINE_SPINLOCK(vmap_area_lock);
>  /* Export for kexec only */
>  LIST_HEAD(vmap_area_list);
> @@ -1108,7 +1106,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>
>         va->va_start = addr;
>         va->va_end = addr + size;
> -       va->flags = 0;
> +       va->vm = NULL;
>         insert_vmap_area(va, &vmap_area_root, &vmap_area_list);
>
>         spin_unlock(&vmap_area_lock);
> @@ -1912,7 +1910,6 @@ void __init vmalloc_init(void)
>                 if (WARN_ON_ONCE(!va))
>                         continue;
>
> -               va->flags = VM_VM_AREA;
>                 va->va_start = (unsigned long)tmp->addr;
>                 va->va_end = va->va_start + tmp->size;
>                 va->vm = tmp;
> @@ -2010,7 +2007,6 @@ static void setup_vmalloc_vm(struct vm_struct *vm, struct vmap_area *va,
>         vm->size = va->va_end - va->va_start;
>         vm->caller = caller;
>         va->vm = vm;
> -       va->flags |= VM_VM_AREA;
>         spin_unlock(&vmap_area_lock);
>  }
>
> @@ -2115,7 +2111,7 @@ struct vm_struct *find_vm_area(const void *addr)
>         struct vmap_area *va;
>
>         va = find_vmap_area((unsigned long)addr);
> -       if (va && va->flags & VM_VM_AREA)
> +       if (va && va->vm)
>                 return va->vm;
>
>         return NULL;
> @@ -2139,11 +2135,10 @@ struct vm_struct *remove_vm_area(const void *addr)
>
>         spin_lock(&vmap_area_lock);
>         va = __find_vmap_area((unsigned long)addr);
> -       if (va && va->flags & VM_VM_AREA) {
> +       if (va && va->vm) {
>                 struct vm_struct *vm = va->vm;
>
>                 va->vm = NULL;
> -               va->flags &= ~VM_VM_AREA;
>                 spin_unlock(&vmap_area_lock);
>
>                 kasan_free_shadow(vm);
> @@ -2854,7 +2849,7 @@ long vread(char *buf, char *addr, unsigned long count)
>                 if (!count)
>                         break;
>
> -               if (!(va->flags & VM_VM_AREA))
> +               if (!va->vm)
>                         continue;
>
>                 vm = va->vm;
> @@ -2934,7 +2929,7 @@ long vwrite(char *buf, char *addr, unsigned long count)
>                 if (!count)
>                         break;
>
> -               if (!(va->flags & VM_VM_AREA))
> +               if (!va->vm)
>                         continue;
>
>                 vm = va->vm;
> @@ -3464,10 +3459,10 @@ static int s_show(struct seq_file *m, void *p)
>         va = list_entry(p, struct vmap_area, list);
>
>         /*
> -        * s_show can encounter race with remove_vm_area, !VM_VM_AREA on
> -        * behalf of vmap area is being tear down or vm_map_ram allocation.
> +        * s_show can encounter race with remove_vm_area, !vm on behalf
> +        * of vmap area is being tear down or vm_map_ram allocation.
>          */
> -       if (!(va->flags & VM_VM_AREA)) {
> +       if (!va->vm) {
>                 seq_printf(m, "0x%pK-0x%pK %7ld vm_map_ram\n",
>                         (void *)va->va_start, (void *)va->va_end,
>                         va->va_end - va->va_start);
> <snip>
>
> urezki@pc636:~/data/ssd/coding/linux-stable$ pahole -C vmap_area mm/vmalloc.o
> die__process_function: tag not supported (INVALID)!
> struct vmap_area {
>         long unsigned int          va_start;             /*     0     8 */
>         long unsigned int          va_end;               /*     8     8 */
>         struct rb_node             rb_node;              /*    16    24 */
>         struct list_head           list;                 /*    40    16 */
>         union {
>                 long unsigned int  subtree_max_size;     /*           8 */
>                 struct llist_node  purge_list;           /*           8 */
>                 struct vm_struct * vm;                   /*           8 */
>         };                                               /*    56     8 */
>         /* --- cacheline 1 boundary (64 bytes) --- */
>
>         /* size: 64, cachelines: 1, members: 5 */
> };
> urezki@pc636:~/data/ssd/coding/linux-stable$
>
> --
> Vlad Rezki