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SubjectRe: [slub p4 2/7] slub: Remove useless statements in __slab_alloc
On Tue, 9 Aug 2011, Christoph Lameter wrote:
> Two statements in __slab_alloc() do not have any effect.
>
> 1. c->page is already set to NULL by deactivate_slab() called right before.
>
> 2. gfpflags are masked in new_slab() before being passed to the page
> allocator. There is no need to mask gfpflags in __slab_alloc in particular
> since most frequent processing in __slab_alloc does not require the use of a
> gfpmask.
>
> Cc: torvalds@linux-foundation.org
> Signed-off-by: Christoph Lameter <cl@linux.com>

Linus wasn't actually on the CC of the email.

> ---
> mm/slub.c | 4 ----
> 1 file changed, 4 deletions(-)
>
> Index: linux-2.6/mm/slub.c
> ===================================================================
> --- linux-2.6.orig/mm/slub.c 2011-08-01 11:03:15.000000000 -0500
> +++ linux-2.6/mm/slub.c 2011-08-01 11:04:06.385859038 -0500
> @@ -2064,9 +2064,6 @@ static void *__slab_alloc(struct kmem_ca
> c = this_cpu_ptr(s->cpu_slab);
> #endif
>
> - /* We handle __GFP_ZERO in the caller */
> - gfpflags &= ~__GFP_ZERO;
> -

This is the part Linus felt strongly about in the past. It's not needed
now that we mask GFP flags in new_slab() and the code path is pretty hot
on workloads where SLAB traditionally has had better performance.

> page = c->page;
> if (!page)
> goto new_slab;
> @@ -2163,7 +2160,6 @@ debug:
>
> c->freelist = get_freepointer(s, object);
> deactivate_slab(s, c);
> - c->page = NULL;
> c->node = NUMA_NO_NODE;
> local_irq_restore(flags);
> return object;
>
>


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