Re: [PATCH v5 2/2] mm/vmalloc: modify struct vmap_area to reduce its size

From: Pengfei Li
Date: Tue Jul 16 2019 - 11:07:25 EST

On Tue, Jul 16, 2019 at 10:35 PM Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxx>
> > Suggested-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > ---
> > 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.
>

Thank you.
I will keep the comment in the next version.

> > - 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