Re: [PATCH v4] mm/kfence: reset PG_slab and memcg_data before freeing __kfence_pool

[Marco Elver]

On Thu, May 05, 2022 at 09:45PM +0900, Hyeonggon Yoo wrote:
> When kfence fails to initialize kfence pool, it frees the pool.
> But it does not reset memcg_data and PG_slab flag.
> 
> Below is a BUG because of this. Let's fix it by resetting memcg_data
> and PG_slab flag 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@xxxxxxxxx>

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

Thank you both.

> ---
> 
> v3 -> v4:
> 	- Use struct slab instead of folio and reset memcg_data and
> 	  clear PG_slab only if PG_slab flag is set (virt_to_slab() != NULL).
> 	  
> 	  Thanks Muchun. Now it looks much better and still works.
> 
> 	- Remove Reviewed-by: tags as the code changed. Please add new
> 	  one if the patch still looks good to you.
> 
>  mm/kfence/core.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index a203747ad2c0..58dfac1828ce 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -642,6 +642,16 @@ 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;
> +#ifdef CONFIG_MEMCG
> +		slab->memcg_data = 0;
> +#endif
> +		__folio_clear_slab(slab_folio(slab));
> +	}
>  	memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
>  	__kfence_pool = NULL;
>  	return false;
> -- 
> 2.32.0