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Date:   Thu, 5 May 2022 20:33:36 +0900
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] mm/kfence: reset PG_slab and memcg_data before
 freeing __kfence_pool
Message-ID: <YnO2EIB6LzaBKEvi@hyeyoo>
References: <20220505073920.1880661-1-42.hyeyoo@gmail.com>
 <20220505101337.1997819-1-42.hyeyoo@gmail.com>
 <YnOs2k0REk9428LA@FVFYT0MHHV2J.usts.net>
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On Thu, May 05, 2022 at 06:54:18PM +0800, Muchun Song wrote:
> On Thu, May 05, 2022 at 07:13:37PM +0900, Hyeonggon Yoo wrote:
> > When kfence fails to initialize kfence pool, it frees the pool.
> > But it does not reset PG_slab flag and memcg_data of struct page.
> > 
> > Below is a BUG because of this. Let's fix it by resetting PG_slab
> > and memcg_data before free.
> > 
> > [    0.089149] BUG: Bad page state in process swapper/0  pfn:3d8e06
> > [    0.089149] page:ffffea46cf638180 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x3d8e06
> > [    0.089150] memcg:ffffffff94a475d1
> > [    0.089150] flags: 0x17ffffc0000200(slab|node=0|zone=2|lastcpupid=0x1fffff)
> > [    0.089151] raw: 0017ffffc0000200 ffffea46cf638188 ffffea46cf638188 0000000000000000
> > [    0.089152] raw: 0000000000000000 0000000000000000 00000000ffffffff ffffffff94a475d1
> > [    0.089152] page dumped because: page still charged to cgroup
> > [    0.089153] Modules linked in:
> > [    0.089153] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G    B   W         5.18.0-rc1+ #965
> > [    0.089154] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
> > [    0.089154] Call Trace:
> > [    0.089155]  <TASK>
> > [    0.089155]  dump_stack_lvl+0x49/0x5f
> > [    0.089157]  dump_stack+0x10/0x12
> > [    0.089158]  bad_page.cold+0x63/0x94
> > [    0.089159]  check_free_page_bad+0x66/0x70
> > [    0.089160]  __free_pages_ok+0x423/0x530
> > [    0.089161]  __free_pages_core+0x8e/0xa0
> > [    0.089162]  memblock_free_pages+0x10/0x12
> > [    0.089164]  memblock_free_late+0x8f/0xb9
> > [    0.089165]  kfence_init+0x68/0x92
> > [    0.089166]  start_kernel+0x789/0x992
> > [    0.089167]  x86_64_start_reservations+0x24/0x26
> > [    0.089168]  x86_64_start_kernel+0xa9/0xaf
> > [    0.089170]  secondary_startup_64_no_verify+0xd5/0xdb
> > [    0.089171]  </TASK>
> > 
> > Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
> > Fixes: 8f0b36497303 ("mm: kfence: fix objcgs vector allocation")
> > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> > Reviewed-by: Marco Elver <elver@google.com>
> > Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> > 
> > v2 -> v3:
> > 	- Add Reviewed-by: tags from Marco and Muchun. Thanks!
> > 	- Initialize folio where it is defined.
> > 
> >  mm/kfence/core.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> > index a203747ad2c0..b7d3a9667f00 100644
> > --- a/mm/kfence/core.c
> > +++ b/mm/kfence/core.c
> > @@ -642,6 +642,14 @@ static bool __init kfence_init_pool_early(void)
> >  	 * fails for the first page, and therefore expect addr==__kfence_pool in
> >  	 * most failure cases.
> >  	 */
> > +	for (char *p = (char *)addr; p < __kfence_pool + KFENCE_POOL_SIZE; p += PAGE_SIZE) {
> > +		struct folio *folio = virt_to_folio(p);
> > +
> 
> After more thinking, I think it is better to use 'struct slab *'
> to define a local variable since we already use this struct
> throughout slab core. What do you think?
>

I think that may not be better.

In the code we're freeing folios (so not going to reuse it again in slab/kfence).
And it may not be Slab depending on why kfence_init_pool() failed.

> Thanks.
> 
> > +		__folio_clear_slab(folio);
> > +#ifdef CONFIG_MEMCG
> > +		folio->memcg_data = 0;
> > +#endif
> > +	}
> >  	memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
> >  	__kfence_pool = NULL;
> >  	return false;
> > -- 
> > 2.32.0
> > 
> > 

-- 
Thanks,
Hyeonggon