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

[Vlastimil Babka]

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@xxxxxxxxxx>
> 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

----8<----
>From 4d97b55c208c611cb01062e0fbf9dbda9f5617d5 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@xxxxxxx>
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@xxxxxxx>
---
 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