Re: [PATCH v5] slab: reset obj_ext when it is not actually valid during freeing

[Vlastimil Babka <vbabka@suse.cz>]

On 10/15/25 18:29, Suren Baghdasaryan wrote:
> On Wed, Oct 15, 2025 at 7:17 AM Hao Ge <hao.ge@linux.dev> wrote:
>>
>> From: Hao Ge <gehao@kylinos.cn>
>>
>> If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
>> But we did not clear it when freeing the slab. Since OBJEXTS_ALLOC_FAIL and
>> MEMCG_DATA_OBJEXTS currently share the same bit position, during the
>> release of the associated folio, a VM_BUG_ON_FOLIO() check in
>> folio_memcg_kmem() is triggered because it was mistakenly assumed that
>> a valid folio->memcg_data was not cleared before freeing the folio.
>>
>> When freeing a slab, we clear slab->obj_exts and reset it to 0
>> if the obj_ext array has been successfully allocated.
>> So let's reset slab->obj_exts to 0 when freeing a slab if
>> the obj_ext array allocated fail to allow them to be returned
>> to the buddy system more smoothly.
>>
>> Signed-off-by: Hao Ge <gehao@kylinos.cn>
>> ---
>> v5: Adopt the simpler solution proposed by Vlastimil;
>>     Many thanks to him

I've massaged the commit log and comments a bit and also realized that
AFAICS we're actually fixing an issue that predates 7612833192d5 ("slab:
Reuse first bit for OBJEXTS_ALLOC_FAIL"). Am I wrong?

----8<----
From 8151384e5baf34db5812ed51e2e463796ab6e973 Mon Sep 17 00:00:00 2001
From: Hao Ge <gehao@kylinos.cn>
Date: Wed, 15 Oct 2025 22:16:42 +0800
Subject: [PATCH] slab: reset slab->obj_ext when freeing and it is
 OBJEXTS_ALLOC_FAIL

If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
But we do not clear it when freeing the slab. Since OBJEXTS_ALLOC_FAIL and
MEMCG_DATA_OBJEXTS currently share the same bit position, during the
release of the associated folio, a VM_BUG_ON_FOLIO() check in
folio_memcg_kmem() is triggered because it was mistakenly assumed that
a valid folio->memcg_data was not cleared before freeing the folio.

Another problem that predates sharing the OBJEXTS_ALLOC_FAIL and
MEMCG_DATA_OBJEXTS bits is that on configurations with
is_check_pages_enabled(), the non-cleared bit in page->memcg_data will
trigger a free_page_is_bad() failure "page still charged to cgroup"

When freeing a slab, we clear slab->obj_exts if the obj_ext array has
been successfully allocated. So let's clear it also when the allocation
has failed.

Fixes: 09c46563ff6d ("codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations")
Link: https://lore.kernel.org/all/20251015141642.700170-1-hao.ge@linux.dev/
Cc: <stable@vger.kernel.org>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 13ae4491136a..a8fcc7e6f25a 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2170,8 +2170,15 @@ static inline void free_slab_obj_exts(struct slab *slab)
 	struct slabobj_ext *obj_exts;
 
 	obj_exts = slab_obj_exts(slab);
-	if (!obj_exts)
+	if (!obj_exts) {
+		/*
+		 * If obj_exts allocation failed, slab->obj_exts is set to
+		 * OBJEXTS_ALLOC_FAIL. In this case, we end up here and should
+		 * clear the flag.
+		 */
+		slab->obj_exts = 0;
 		return;
+	}
 
 	/*
 	 * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its
-- 
2.51.0