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SubjectRe: [PATCH v5 2/2] mm/vmalloc: modify struct vmap_area to reduce its size
On Tue, Jul 16, 2019 at 09:26:04PM +0800, Pengfei Li wrote:
> Objective
> ---------
> The current implementation of struct vmap_area wasted space.
>
> After applying this commit, sizeof(struct vmap_area) has been
> reduced from 11 words to 8 words.
>
> Description
> -----------
> 1) Pack "subtree_max_size", "vm" and "purge_list".
> This is no problem because
> A) "subtree_max_size" is only used when vmap_area is in
> "free" tree
> B) "vm" is only used when vmap_area is in "busy" tree
> C) "purge_list" is only used when vmap_area is in
> vmap_purge_list
>
> 2) Eliminate "flags".
> Since only one flag VM_VM_AREA is being used, and the same
> thing can be done by judging whether "vm" is NULL, then the
> "flags" can be eliminated.
>
> Signed-off-by: Pengfei Li <lpf.vector@gmail.com>
> Suggested-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
> include/linux/vmalloc.h | 20 +++++++++++++-------
> mm/vmalloc.c | 24 ++++++++++--------------
> 2 files changed, 23 insertions(+), 21 deletions(-)
>
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 9b21d0047710..a1334bd18ef1 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -51,15 +51,21 @@ 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;
> +
> + /*
> + * The following three variables can be packed, because
> + * a vmap_area object is always one of the three states:
> + * 1) in "free" tree (root is vmap_area_root)
> + * 2) in "busy" tree (root is free_vmap_area_root)
> + * 3) in purge list (head is vmap_purge_list)
> + */
> + union {
> + unsigned long subtree_max_size; /* in "free" tree */
> + struct vm_struct *vm; /* in "busy" tree */
> + struct llist_node purge_list; /* in purge list */
> + };
> };
>
> /*
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 71d8040a8a0b..39bf9cf4175a 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -329,7 +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 */
> @@ -1115,7 +1114,7 @@ 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);
> @@ -1922,7 +1921,6 @@ 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;
> @@ -2020,7 +2018,6 @@ 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);
> }
>
> @@ -2125,10 +2122,10 @@ 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)
> - return va->vm;
> + if (!va)
> + return NULL;
>
> - return NULL;
> + return va->vm;
> }
>
> /**
> @@ -2149,11 +2146,10 @@ 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);
> @@ -2856,7 +2852,7 @@ 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;
> @@ -2936,7 +2932,7 @@ 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;
> @@ -3466,10 +3462,10 @@ 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.
> + * If !va->vm then this vmap_area object is allocated
> + * by vm_map_ram.
> */
This point is still valid. There is a race between remove_vm_area() vs
s_show() and va->vm = NULL. So, please keep that comment.

> - 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);
> --
> 2.21.0
>

--
Vlad Rezki

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