Re: poison_element vs highmem, was Re: [linux-next:master] [block] ec7f31b2a2: BUG:unable_to_handle_page_fault_for_address

[Oliver Sang <oliver.sang@intel.com>]

hi, Vlastimil Babka,

On Wed, Nov 12, 2025 at 10:33:32AM +0100, Vlastimil Babka wrote:
> On 11/11/25 08:48, Christoph Hellwig wrote:
> > Looks like this is due to the code in poison_element, which tries
> > to memset more than PAGE_SIZE for a single page.  This probably
> > implies we are the first users of the mempool page helpers for order > 0,
> > or at least the first one tested by anyone on 32-bit with highmem :)
> > 
> > That code seems to come from
> > 
> > commit bdfedb76f4f5aa5e37380e3b71adee4a39f30fc6
> > Author: David Rientjes <rientjes@google.com>
> > Date:   Wed Apr 15 16:14:17 2015 -0700
> > 
> >     mm, mempool: poison elements backed by slab allocator
> > 
> > originally.  The easiest fix would be to just skip poisoning for this
> > case, although that would reduce the usefulness of the poisoning.
> 
> #syz test

we applied below patch upon ec7f31b2a2 directly, and confirmed the issue we
reported gone now with the patch.

Tested-by: kernel test robot <oliver.sang@intel.com>

BTW, we are kernel test robot, not the syzbot :) thanks

> 
> ----8<----
> From 4d97b55c208c611cb01062e0fbf9dbda9f5617d5 Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@suse.cz>
> Date: Wed, 12 Nov 2025 10:29:52 +0100
> Subject: [PATCH] mm/mempool: fix poisoning order>0 pages with HIGHMEM
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/mempool.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/mempool.c b/mm/mempool.c
> index 1c38e873e546..75fea9441b93 100644
> --- a/mm/mempool.c
> +++ b/mm/mempool.c
> @@ -68,10 +68,18 @@ static void check_element(mempool_t *pool, void *element)
>  	} else if (pool->free == mempool_free_pages) {
>  		/* Mempools backed by page allocator */
>  		int order = (int)(long)pool->pool_data;
> -		void *addr = kmap_local_page((struct page *)element);
> +#ifdef CONFIG_HIGHMEM
> +		for (int i = 0; i < (1 << order); i++) {
> +			struct page *page = (struct page *)element;
> +			void *addr = kmap_local_page(page + i);
> 
> -		__check_element(pool, addr, 1UL << (PAGE_SHIFT + order));
> -		kunmap_local(addr);
> +			__check_element(pool, addr, PAGE_SIZE);
> +			kunmap_local(addr);
> +		}
> +#else
> +		void *addr = page_address((struct page *)element);
> +		__check_element(pool, addr, PAGE_SIZE << order);
> +#endif
>  	}
>  }
> 
> @@ -97,10 +105,18 @@ static void poison_element(mempool_t *pool, void *element)
>  	} else if (pool->alloc == mempool_alloc_pages) {
>  		/* Mempools backed by page allocator */
>  		int order = (int)(long)pool->pool_data;
> -		void *addr = kmap_local_page((struct page *)element);
> +#ifdef CONFIG_HIGHMEM
> +		for (int i = 0; i < (1 << order); i++) {
> +			struct page *page = (struct page *)element;
> +			void *addr = kmap_local_page(page + i);
> 
> -		__poison_element(addr, 1UL << (PAGE_SHIFT + order));
> -		kunmap_local(addr);
> +			__poison_element(addr, PAGE_SIZE);
> +			kunmap_local(addr);
> +		}
> +#else
> +		void *addr = page_address((struct page *)element);
> +		__poison_element(addr, PAGE_SIZE << order);
> +#endif
>  	}
>  }
>  #else /* CONFIG_SLUB_DEBUG_ON */
> -- 
> 2.51.1
> 
>