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Message-ID: <4F547C90.4040007@lge.com>
Date: Mon, 05 Mar 2012 17:42:56 +0900
From: Namhyung Kim <namhyung.kim@....com>
To: Minchan Kim <minchan@...nel.org>
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
2012-03-04 7:34 PM, Minchan Kim wrote:
> Hi Namhyung,
>
Hi Minchan,
glad to see you here again :)
> 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?
>
To be honest, I don't have a real test environment which brings this issue in
the first place. On my simple test environment, enabling CONFIG_DEBUG_VM emits
a bug when I tried to free middle of the slab pages. Thanks for pointing it out.
However I guess there was a chance to bypass that test anyhow since it did
reach to __free_pages_ok(). If the page count was 0 already, free_pages() will
prevent it from getting to the function even though CONFIG_DEBUG_VM was
disabled. But I don't think it's a kernel bug - it seems entirely our fault :(
I'll recheck and talk about it with my colleagues.
Thanks,
Namhyung
>> ---
>> 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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>> see: http://www.linux-mm.org/ .
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>
> --
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