linux-kernel - Re: [PATCH -next] slub: set PG

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Date:	Sun, 4 Mar 2012 19:34:46 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Namhyung Kim <namhyung.kim@....com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Namhyung Kim <namhyung@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] slub: set PG_slab on all of slab pages

Hi Namhyung,

On Wed, Feb 29, 2012 at 05:54:34PM +0900, Namhyung Kim wrote:
> Unlike SLAB, SLUB doesn't set PG_slab on tail pages, so if a user would
> call free_pages() incorrectly on a object in a tail page, she will get
>i confused with the undefined result. Setting the flag would help her by
> emitting a warning on bad_page() in such a case.
> 
> Reported-by: Sangseok Lee <sangseok.lee@....com>
> Signed-off-by: Namhyung Kim <namhyung.kim@....com>

I read this thread and I feel the we don't reach right point.
I think it's not a compound page problem.
We can face above problem where we allocates big order page without __GFP_COMP
and free middle page of it.

Fortunately, We can catch such a problem by put_page_testzero in __free_pages
if you enable CONFIG_DEBUG_VM.

Did you tried that with CONFIG_DEBUG_VM?

> ---
>  mm/slub.c |   12 ++++++++++--
>  1 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 33bab2aca882..575baacbec9b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1287,6 +1287,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  	struct page *page;
>  	struct kmem_cache_order_objects oo = s->oo;
>  	gfp_t alloc_gfp;
> +	int i;
>  
>  	flags &= gfp_allowed_mask;
>  
> @@ -1320,6 +1321,9 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  	if (!page)
>  		return NULL;
>  
> +	for (i = 0; i < 1 << oo_order(oo); i++)
> +		__SetPageSlab(page + i);
> +
>  	if (kmemcheck_enabled
>  		&& !(s->flags & (SLAB_NOTRACK | DEBUG_DEFAULT_FLAGS))) {
>  		int pages = 1 << oo_order(oo);
> @@ -1369,7 +1373,6 @@ static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int node)
>  
>  	inc_slabs_node(s, page_to_nid(page), page->objects);
>  	page->slab = s;
> -	page->flags |= 1 << PG_slab;
>  
>  	start = page_address(page);
>  
> @@ -1396,6 +1399,7 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
>  {
>  	int order = compound_order(page);
>  	int pages = 1 << order;
> +	int i;
>  
>  	if (kmem_cache_debug(s)) {
>  		void *p;
> @@ -1413,7 +1417,11 @@ static void __free_slab(struct kmem_cache *s, struct page *page)
>  		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE,
>  		-pages);
>  
> -	__ClearPageSlab(page);
> +	for (i = 0; i < pages; i++) {
> +		BUG_ON(!PageSlab(page + i));
> +		__ClearPageSlab(page + i);
> +	}
> +
>  	reset_page_mapcount(page);
>  	if (current->reclaim_state)
>  		current->reclaim_state->reclaimed_slab += pages;
> -- 
> 1.7.9
> 
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