linux-kernel - Re: [PATCH v2] mm: kfence: fix objcgs vector allocation

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Message-ID: <YkILyqc1WIfQLCTI@carbon.dhcp.thefacebook.com>
Date:   Mon, 28 Mar 2022 12:26:02 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     glider@...gle.com, elver@...gle.com, dvyukov@...gle.com,
        akpm@...ux-foundation.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        duanxiongchun@...edance.com
Subject: Re: [PATCH v2] mm: kfence: fix objcgs vector allocation

On Mon, Mar 28, 2022 at 09:28:43PM +0800, Muchun Song wrote:
> If the kfence object is allocated to be used for objects vector, then
> this slot of the pool eventually being occupied permanently since
> the vector is never freed.  The solutions could be 1) freeing vector
> when the kfence object is freed or 2) allocating all vectors statically.
> Since the memory consumption of object vectors is low, it is better to
> chose 2) to fix the issue and it is also can reduce overhead of vectors
> allocating in the future.
> 
> Fixes: d3fb45f370d9 ("mm, kfence: insert KFENCE hooks for SLAB")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
> v2:
>  - Fix compiler error reported by kernel test robot <lkp@...el.com>.

Reviewed-by: Roman Gushchin <roman.gushchin@...ux.dev>

LGTM, thanks!


> 
>  mm/kfence/core.c   | 11 ++++++++++-
>  mm/kfence/kfence.h |  3 +++
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 13128fa13062..d4c7978cd75e 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -555,6 +555,8 @@ static bool __init kfence_init_pool(void)
>  	 * enters __slab_free() slow-path.
>  	 */
>  	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
> +		struct slab *slab = page_slab(&pages[i]);
> +
>  		if (!i || (i % 2))
>  			continue;
>  
> @@ -562,7 +564,11 @@ static bool __init kfence_init_pool(void)
>  		if (WARN_ON(compound_head(&pages[i]) != &pages[i]))
>  			goto err;
>  
> -		__SetPageSlab(&pages[i]);
> +		__folio_set_slab(slab_folio(slab));
> +#ifdef CONFIG_MEMCG
> +		slab->memcg_data = (unsigned long)&kfence_metadata[i / 2 - 1].objcg |
> +				   MEMCG_DATA_OBJCGS;
> +#endif

We can probably put CONFIG_MEMCG_KMEM here, but it doesn't matter that much.
In the long run we should get rid of CONFIG_MEMCG_KMEM anyway.