Re: [PATCH -mmotm 05/30] mm: sl[au]b: add knowledge of reserve pages

[Pekka Enberg <penberg@cs.helsinki.fi>]

Hi Xiaotian!

I would actually prefer that the SLAB, SLOB, and SLUB changes were in
separate patches to make reviewing easier.

Looking at SLUB:

On Tue, Jul 13, 2010 at 1:17 PM, Xiaotian Feng <dfeng@redhat.com> wrote:
> diff --git a/mm/slub.c b/mm/slub.c
> index 7bb7940..7a5d6dc 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -27,6 +27,8 @@
>  #include <linux/memory.h>
>  #include <linux/math64.h>
>  #include <linux/fault-inject.h>
> +#include "internal.h"
> +
>
>  /*
>  * Lock order:
> @@ -1139,7 +1141,8 @@ static void setup_object(struct kmem_cache *s, struct page *page,
>                s->ctor(object);
>  }
>
> -static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
> +static
> +struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node, int *reserve)
>  {
>        struct page *page;
>        void *start;
> @@ -1153,6 +1156,8 @@ static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
>        if (!page)
>                goto out;
>
> +       *reserve = page->reserve;
> +
>        inc_slabs_node(s, page_to_nid(page), page->objects);
>        page->slab = s;
>        page->flags |= 1 << PG_slab;
> @@ -1606,10 +1611,20 @@ static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>  {
>        void **object;
>        struct page *new;
> +       int reserve;
>
>        /* We handle __GFP_ZERO in the caller */
>        gfpflags &= ~__GFP_ZERO;
>
> +       if (unlikely(c->reserve)) {
> +               /*
> +                * If the current slab is a reserve slab and the current
> +                * allocation context does not allow access to the reserves we
> +                * must force an allocation to test the current levels.
> +                */
> +               if (!(gfp_to_alloc_flags(gfpflags) & ALLOC_NO_WATERMARKS))
> +                       goto grow_slab;

OK, so assume that:

  (1) c->reserve is set to one

  (2) GFP flags don't allow dipping into the reserves

  (3) we've managed to free enough pages so normal
       allocations are fine

  (4) the page from reserves is not yet empty

we will call flush_slab() and put the "emergency page" on partial list
and clear c->reserve. This effectively means that now some other
allocation can fetch the partial page and start to use it. Is this OK?
Who makes sure the emergency reserves are large enough for the next
out-of-memory condition where we swap over NFS?

> +       }
>        if (!c->page)
>                goto new_slab;
>
> @@ -1623,8 +1638,8 @@ load_freelist:
>        object = c->page->freelist;
>        if (unlikely(!object))
>                goto another_slab;
> -       if (unlikely(SLABDEBUG && PageSlubDebug(c->page)))
> -               goto debug;
> +       if (unlikely(SLABDEBUG && PageSlubDebug(c->page) || c->reserve))
> +               goto slow_path;
>
>        c->freelist = get_freepointer(s, object);
>        c->page->inuse = c->page->objects;
> @@ -1646,16 +1661,18 @@ new_slab:
>                goto load_freelist;
>        }
>
> +grow_slab:
>        if (gfpflags & __GFP_WAIT)
>                local_irq_enable();
>
> -       new = new_slab(s, gfpflags, node);
> +       new = new_slab(s, gfpflags, node, &reserve);
>
>        if (gfpflags & __GFP_WAIT)
>                local_irq_disable();
>
>        if (new) {
>                c = __this_cpu_ptr(s->cpu_slab);
> +               c->reserve = reserve;
>                stat(s, ALLOC_SLAB);
>                if (c->page)
>                        flush_slab(s, c);
> @@ -1667,10 +1684,20 @@ new_slab:
>        if (!(gfpflags & __GFP_NOWARN) && printk_ratelimit())
>                slab_out_of_memory(s, gfpflags, node);
>        return NULL;
> -debug:
> -       if (!alloc_debug_processing(s, c->page, object, addr))
> +
> +slow_path:
> +       if (!c->reserve && !alloc_debug_processing(s, c->page, object, addr))
>                goto another_slab;
>
> +       /*
> +        * Avoid the slub fast path in slab_alloc() by not setting
> +        * c->freelist and the fast path in slab_free() by making
> +        * node_match() fail by setting c->node to -1.
> +        *
> +        * We use this for for debug and reserve checks which need
> +        * to be done for each allocation.
> +        */
> +
>        c->page->inuse++;
>        c->page->freelist = get_freepointer(s, object);
>        c->node = -1;
> @@ -2095,10 +2122,11 @@ static void early_kmem_cache_node_alloc(gfp_t gfpflags, int node)
>        struct page *page;
>        struct kmem_cache_node *n;
>        unsigned long flags;
> +       int reserve;
>
>        BUG_ON(kmalloc_caches->size < sizeof(struct kmem_cache_node));
>
> -       page = new_slab(kmalloc_caches, gfpflags, node);
> +       page = new_slab(kmalloc_caches, gfpflags, node, &reserve);
>
>        BUG_ON(!page);
>        if (page_to_nid(page) != node) {
> --
> 1.7.1.1
>
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