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

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

On Tue, Jul 16, 2019 at 10:35 PM Uladzislau Rezki <urezki@gmail.com> 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@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.
>

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