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

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Message-ID: <d64db373-cb98-4142-ba44-1aa5ccbdcb45@suse.cz>
Date: Thu, 16 Oct 2025 15:18:36 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, Hao Ge <hao.ge@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter
 <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Alexei Starovoitov <ast@...nel.org>, Shakeel Butt <shakeel.butt@...ux.dev>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, Hao Ge <gehao@...inos.cn>
Subject: Re: [PATCH v5] slab: reset obj_ext when it is not actually valid
 during freeing

On 10/16/25 15:09, Harry Yoo wrote:
>> ----8<----
>> From 8151384e5baf34db5812ed51e2e463796ab6e973 Mon Sep 17 00:00:00 2001
>> From: Hao Ge <gehao@...inos.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.
> 
> nit: maybe this can be massaged as "a VM_BUG_ON_FOLIO() check in folio_memcg_kmem()
> is triggered because the OBJEXTS_ALLOC_FAIL flag was not cleared,
> causing it to be interpreted as a kmem folio (non-slab) with
> MEMCG_OBJEXTS_DATA flag set, which is invalid because MEMCG_OBJEXTS_DATA
> is supposed to be set only on slabs."

Thanks, applied

>> 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@...r.kernel.org>
>> Signed-off-by: Hao Ge <gehao@...inos.cn>
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>> ---
> 
> Looks good to me,
> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

and this too.