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Date: Mon, 10 Jul 2023 12:21:30 +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, Jul 10, 2023 at 12:19PM +0200, Marco Elver wrote:
> 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.
Forgot to attach .config -- attached config.
All I see is:
[ 0.303465] rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
[ 0.304783] rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=8
[ 0.316800] NR_IRQS: 4352, nr_irqs: 488, preallocated irqs: 16
[ 0.318140] rcu: srcu_init: Setting srcu_struct sizes based on contention.
[ 0.320001] kfence: kfence_init failed
[ 0.326880] Console: colour VGA+ 80x25
[ 0.327585] printk: console [ttyS0] enabled
[ 0.327585] printk: console [ttyS0] enabled
around KFENCE initialization.
> > ---
> > 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.
View attachment ".config" of type "text/plain" (142262 bytes)
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