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Message-ID: <CANpmjNOHz+dRbJsAyg29nksPMcd2P6109iPxTem_-b2qfUvXtw@mail.gmail.com>
Date: Mon, 10 Jul 2023 12:19:49 +0200
From: Marco Elver <elver@...gle.com>
To: Peng Zhang <zhangpeng.00@...edance.com>
Cc: glider@...gle.com, dvyukov@...gle.com, akpm@...ux-foundation.org,
kasan-dev@...glegroups.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, muchun.song@...ux.dev
Subject: Re: [PATCH] mm: kfence: allocate kfence_metadata at runtime
On Mon, 10 Jul 2023 at 05:27, 'Peng Zhang' via kasan-dev
<kasan-dev@...glegroups.com> wrote:
>
> kfence_metadata is currently a static array. For the purpose of
> allocating scalable __kfence_pool, we first change it to runtime
> allocation of metadata. Since the size of an object of kfence_metadata
> is 1160 bytes, we can save at least 72 pages (with default 256 objects)
> without enabling kfence.
>
> Below is the numbers obtained in qemu (with default 256 objects).
> before: Memory: 8134692K/8388080K available (3668K bss)
> after: Memory: 8136740K/8388080K available (1620K bss)
> More than expected, it saves 2MB memory.
>
> Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
Seems like a reasonable optimization, but see comments below.
Also with this patch applied on top of v6.5-rc1, KFENCE just doesn't
init at all anymore (early init). Please fix.
> ---
> mm/kfence/core.c | 102 ++++++++++++++++++++++++++++++++-------------
> mm/kfence/kfence.h | 5 ++-
> 2 files changed, 78 insertions(+), 29 deletions(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index dad3c0eb70a0..b9fec1c46e3d 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -116,7 +116,7 @@ EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
> * backing pages (in __kfence_pool).
> */
> static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
> -struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
> +struct kfence_metadata *kfence_metadata;
>
> /* Freelist with available objects. */
> static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
> @@ -643,13 +643,56 @@ static unsigned long kfence_init_pool(void)
> return addr;
> }
>
> +static int kfence_alloc_metadata(void)
> +{
> + unsigned long nr_pages = KFENCE_METADATA_SIZE / PAGE_SIZE;
> +
> +#ifdef CONFIG_CONTIG_ALLOC
> + struct page *pages;
> +
> + pages = alloc_contig_pages(nr_pages, GFP_KERNEL, first_online_node,
> + NULL);
> + if (pages)
> + kfence_metadata = page_to_virt(pages);
> +#else
> + if (nr_pages > MAX_ORDER_NR_PAGES) {
> + pr_warn("KFENCE_NUM_OBJECTS too large for buddy allocator\n");
Does this mean that KFENCE won't work at all if we can't allocate the
metadata? I.e. it won't work either in early nor late init modes?
I know we already have this limitation for _late init_ of the KFENCE pool.
So I have one major question: when doing _early init_, what is the
maximum size of the KFENCE pool (#objects) with this change?
> + return -EINVAL;
> + }
> + kfence_metadata = alloc_pages_exact(KFENCE_METADATA_SIZE,
> + GFP_KERNEL);
> +#endif
> +
> + if (!kfence_metadata)
> + return -ENOMEM;
> +
> + memset(kfence_metadata, 0, KFENCE_METADATA_SIZE);
memzero_explicit, or pass __GFP_ZERO to alloc_pages?
> + return 0;
> +}
> +
> +static void kfence_free_metadata(void)
> +{
> + if (WARN_ON(!kfence_metadata))
> + return;
> +#ifdef CONFIG_CONTIG_ALLOC
> + free_contig_range(page_to_pfn(virt_to_page((void *)kfence_metadata)),
> + KFENCE_METADATA_SIZE / PAGE_SIZE);
> +#else
> + free_pages_exact((void *)kfence_metadata, KFENCE_METADATA_SIZE);
> +#endif
> + kfence_metadata = NULL;
> +}
> +
> static bool __init kfence_init_pool_early(void)
> {
> - unsigned long addr;
> + unsigned long addr = (unsigned long)__kfence_pool;
>
> if (!__kfence_pool)
> return false;
>
> + if (!kfence_alloc_metadata())
> + goto free_pool;
> +
> addr = kfence_init_pool();
>
> if (!addr) {
> @@ -663,6 +706,7 @@ static bool __init kfence_init_pool_early(void)
> return true;
> }
>
> + kfence_free_metadata();
> /*
> * Only release unprotected pages, and do not try to go back and change
> * page attributes due to risk of failing to do so as well. If changing
> @@ -670,31 +714,12 @@ static bool __init kfence_init_pool_early(void)
> * fails for the first page, and therefore expect addr==__kfence_pool in
> * most failure cases.
> */
> +free_pool:
> memblock_free_late(__pa(addr), KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool));
> __kfence_pool = NULL;
> return false;
> }
>
> -static bool kfence_init_pool_late(void)
> -{
> - unsigned long addr, free_size;
> -
> - addr = kfence_init_pool();
> -
> - if (!addr)
> - return true;
> -
> - /* Same as above. */
> - free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
> -#ifdef CONFIG_CONTIG_ALLOC
> - free_contig_range(page_to_pfn(virt_to_page((void *)addr)), free_size / PAGE_SIZE);
> -#else
> - free_pages_exact((void *)addr, free_size);
> -#endif
> - __kfence_pool = NULL;
> - return false;
> -}
> -
> /* === DebugFS Interface ==================================================== */
>
> static int stats_show(struct seq_file *seq, void *v)
> @@ -896,6 +921,10 @@ void __init kfence_init(void)
> static int kfence_init_late(void)
> {
> const unsigned long nr_pages = KFENCE_POOL_SIZE / PAGE_SIZE;
> + unsigned long addr = (unsigned long)__kfence_pool;
> + unsigned long free_size = KFENCE_POOL_SIZE;
> + int ret;
> +
> #ifdef CONFIG_CONTIG_ALLOC
> struct page *pages;
>
> @@ -913,15 +942,29 @@ static int kfence_init_late(void)
> return -ENOMEM;
> #endif
>
> - if (!kfence_init_pool_late()) {
> - pr_err("%s failed\n", __func__);
> - return -EBUSY;
> + ret = kfence_alloc_metadata();
> + if (!ret)
> + goto free_pool;
> +
> + addr = kfence_init_pool();
> + if (!addr) {
> + kfence_init_enable();
> + kfence_debugfs_init();
> + return 0;
> }
>
> - kfence_init_enable();
> - kfence_debugfs_init();
> + pr_err("%s failed\n", __func__);
> + kfence_free_metadata();
> + free_size = KFENCE_POOL_SIZE - (addr - (unsigned long)__kfence_pool);
> + ret = -EBUSY;
>
> - return 0;
> +free_pool:
> +#ifdef CONFIG_CONTIG_ALLOC
> + free_contig_range(page_to_pfn(virt_to_page((void *)addr)), free_size / PAGE_SIZE);
> +#else
> + free_pages_exact((void *)addr, free_size);
> +#endif
You moved this from kfence_init_pool_late - that did "__kfence_pool =
NULL" which is missing now.
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