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Date:   Fri, 15 Sep 2023 13:55:43 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Matteo Rizzo <matteorizzo@google.com>
Cc:     cl@linux.com, penberg@kernel.org, rientjes@google.com,
        iamjoonsoo.kim@lge.com, akpm@linux-foundation.org, vbabka@suse.cz,
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        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-mm@kvack.org, linux-hardening@vger.kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com,
        corbet@lwn.net, luto@kernel.org, peterz@infradead.org,
        jannh@google.com, evn@google.com, poprdi@google.com,
        jordyzomer@google.com
Subject: Re: [RFC PATCH 02/14] mm/slub: add is_slab_addr/is_slab_page helpers
Message-ID: <202309151352.9803BA1@keescook>
References: <20230915105933.495735-1-matteorizzo@google.com>
 <20230915105933.495735-3-matteorizzo@google.com>
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On Fri, Sep 15, 2023 at 10:59:21AM +0000, Matteo Rizzo wrote:
> From: Jann Horn <jannh@google.com>
> 
> This is refactoring in preparation for adding two different
> implementations (for SLAB_VIRTUAL enabled and disabled).
> 
> virt_to_folio(x) expands to _compound_head(virt_to_page(x)) and
> virt_to_head_page(x) also expands to _compound_head(virt_to_page(x))
> 
> so PageSlab(virt_to_head_page(res)) should be equivalent to
> is_slab_addr(res).

Perhaps add a note that redundant calls to virt_to_folio() will be
removed in following patches?

> 
> Signed-off-by: Jann Horn <jannh@google.com>
> Co-developed-by: Matteo Rizzo <matteorizzo@google.com>
> Signed-off-by: Matteo Rizzo <matteorizzo@google.com>
> ---
>  include/linux/slab.h | 1 +
>  kernel/resource.c    | 2 +-
>  mm/slab.h            | 9 +++++++++
>  mm/slab_common.c     | 5 ++---
>  mm/slub.c            | 6 +++---
>  5 files changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 8228d1276a2f..a2d82010d269 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -793,4 +793,5 @@ int slab_dead_cpu(unsigned int cpu);
>  #define slab_dead_cpu		NULL
>  #endif
>  
> +#define is_slab_addr(addr) folio_test_slab(virt_to_folio(addr))
>  #endif	/* _LINUX_SLAB_H */
> diff --git a/kernel/resource.c b/kernel/resource.c
> index b1763b2fd7ef..c829e5f97292 100644
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -158,7 +158,7 @@ static void free_resource(struct resource *res)
>  	 * buddy and trying to be smart and reusing them eventually in
>  	 * alloc_resource() overcomplicates resource handling.
>  	 */
> -	if (res && PageSlab(virt_to_head_page(res)))
> +	if (res && is_slab_addr(res))
>  		kfree(res);
>  }
>  
> diff --git a/mm/slab.h b/mm/slab.h
> index 799a315695c6..25e41dd6087e 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -169,6 +169,15 @@ static_assert(IS_ALIGNED(offsetof(struct slab, freelist), sizeof(freelist_aba_t)
>   */
>  #define slab_page(s) folio_page(slab_folio(s), 0)
>  
> +/**
> + * is_slab_page - Checks if a page is really a slab page
> + * @s: The slab
> + *
> + * Checks if s points to a slab page.
> + *
> + * Return: true if s points to a slab and false otherwise.
> + */
> +#define is_slab_page(s) folio_test_slab(slab_folio(s))
>  /*
>   * If network-based swap is enabled, sl*b must keep track of whether pages
>   * were allocated from pfmemalloc reserves.
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index e99e821065c3..79102d24f099 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1063,7 +1063,7 @@ void kfree(const void *object)
>  		return;
>  
>  	folio = virt_to_folio(object);
> -	if (unlikely(!folio_test_slab(folio))) {
> +	if (unlikely(!is_slab_addr(object))) {
>  		free_large_kmalloc(folio, (void *)object);
>  		return;
>  	}
> @@ -1094,8 +1094,7 @@ size_t __ksize(const void *object)
>  		return 0;
>  
>  	folio = virt_to_folio(object);
> -
> -	if (unlikely(!folio_test_slab(folio))) {
> +	if (unlikely(!is_slab_addr(object))) {
>  		if (WARN_ON(folio_size(folio) <= KMALLOC_MAX_CACHE_SIZE))
>  			return 0;
>  		if (WARN_ON(object != folio_address(folio)))

In the above 2 hunks we're doing virt_to_folio() twice, but I see in
patch 4 that these go away.

> diff --git a/mm/slub.c b/mm/slub.c
> index a7dae207c2d2..b69916ab7aa8 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1259,7 +1259,7 @@ static int check_slab(struct kmem_cache *s, struct slab *slab)
>  {
>  	int maxobj;
>  
> -	if (!folio_test_slab(slab_folio(slab))) {
> +	if (!is_slab_page(slab)) {
>  		slab_err(s, slab, "Not a valid slab page");
>  		return 0;
>  	}
> @@ -1454,7 +1454,7 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
>  	return true;
>  
>  bad:
> -	if (folio_test_slab(slab_folio(slab))) {
> +	if (is_slab_page(slab)) {
>  		/*
>  		 * If this is a slab page then lets do the best we can
>  		 * to avoid issues in the future. Marking all objects
> @@ -1484,7 +1484,7 @@ static inline int free_consistency_checks(struct kmem_cache *s,
>  		return 0;
>  
>  	if (unlikely(s != slab->slab_cache)) {
> -		if (!folio_test_slab(slab_folio(slab))) {
> +		if (!is_slab_page(slab)) {
>  			slab_err(s, slab, "Attempt to free object(0x%p) outside of slab",
>  				 object);
>  		} else if (!slab->slab_cache) {
> -- 
> 2.42.0.459.ge4e396fd5e-goog

This all looks nice and mechanical. :P

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook