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Date:   Thu, 5 May 2022 20:00:41 +0800
From:   Muchun Song <songmuchun@bytedance.com>
To:     Hyeonggon Yoo <42.hyeyoo@gmail.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: <YnO8ac3wxgcZV1sE@FVFYT0MHHV2J.usts.net>
References: <20220505073920.1880661-1-42.hyeyoo@gmail.com>
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 <YnOs2k0REk9428LA@FVFYT0MHHV2J.usts.net>
 <YnO2EIB6LzaBKEvi@hyeyoo>
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On Thu, May 05, 2022 at 08:33:36PM +0900, Hyeonggon Yoo wrote:
> 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.
>

If it it not a Slab, then virt_to_slab() returns NULL in this case,
it is unnecessary to clear PG_slab and reset its ->memcg_data. Right?

Like the following changes:

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 6e69986c3f0d..d90fe82dc752 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -627,6 +627,16 @@ 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 slab *slab = virt_to_slab(p);
+
+               if (!slab)
+                       continue;
+               __folio_clear_slab(slab_folio(slab));
+#ifdef CONFIG_MEMCG
+               slab->memcg_data = 0;
+#endif
+       }
        memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
        __kfence_pool = NULL;
        return false;