linux-kernel - Re: [PATCH 1/2] slab: do ClearSlabPfmemalloc() for all pages of slab

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Date:	Mon, 3 Sep 2012 11:08:55 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Joonsoo Kim <js1304@...il.com>
Cc:	Pekka Enberg <penberg@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH 1/2] slab:  do ClearSlabPfmemalloc() for all pages of slab

It took me a while to getting around to reviewing this due to attending
kernel summit. Sorry about that.

On Sat, Aug 25, 2012 at 11:11:10PM +0900, Joonsoo Kim wrote:
> Now, we just do ClearSlabPfmemalloc() for first page of slab
> when we clear SlabPfmemalloc flag. It is a problem because we sometimes
> test flag of page which is not first page of slab in __ac_put_obj().
> 

Well spotted.

The impact is marginal as far as pfmemalloc protection is concerned. I do not
believe that any of the slabs that use high-order allocations are used in for
the swap-over-network paths. It would be unfortunate if that ever changed.

> So add code to do ClearSlabPfmemalloc for all pages of slab.
> 

I would prefer if the pfmemalloc information was kept on the head page.
Would the following patch also address your concerns?

diff --git a/mm/slab.c b/mm/slab.c
index 811af03..d34a903 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1000,7 +1000,7 @@ static void *__ac_get_obj(struct kmem_cache *cachep, struct array_cache *ac,
 		l3 = cachep->nodelists[numa_mem_id()];
 		if (!list_empty(&l3->slabs_free) && force_refill) {
 			struct slab *slabp = virt_to_slab(objp);
-			ClearPageSlabPfmemalloc(virt_to_page(slabp->s_mem));
+			ClearPageSlabPfmemalloc(virt_to_head_page(slabp->s_mem));
 			clear_obj_pfmemalloc(&objp);
 			recheck_pfmemalloc_active(cachep, ac);
 			return objp;
@@ -1032,7 +1032,7 @@ static void *__ac_put_obj(struct kmem_cache *cachep, struct array_cache *ac,
 {
 	if (unlikely(pfmemalloc_active)) {
 		/* Some pfmemalloc slabs exist, check if this is one */
-		struct page *page = virt_to_page(objp);
+		struct page *page = virt_to_head_page(objp);
 		if (PageSlabPfmemalloc(page))
 			set_obj_pfmemalloc(&objp);
 	}

-- 
Mel Gorman
SUSE Labs
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