linux-kernel - Re: [PATCH v2] mm/kfence: reset PG_slab and memcg_data before freeing __kfence

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Date:   Thu, 5 May 2022 10:25:18 +0200
From:   Marco Elver <elver@...gle.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/kfence: reset PG_slab and memcg_data before freeing __kfence_pool

On Thu, 5 May 2022 at 09:40, Hyeonggon Yoo <42.hyeyoo@...il.com> 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@...il.com>

Reviewed-by: Marco Elver <elver@...gle.com>

, with small comment below.

> ---
>
> v1 -> v2:
>         - Use folio instead of page
>         - Add Fixes: tags
>         - Wrap #ifdef ~ #endif around folio->memcg_data = 0;
>
>  mm/kfence/core.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index a203747ad2c0..bb1c6c489d0a 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -642,6 +642,15 @@ 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;
> +
> +               folio = virt_to_folio(p);

Assign folio where it is defined above. Better to always initialize
and guard against accidental uninit use in future.

Thanks,
-- Marco