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Date: Sun, 30 Oct 2022 03:59:22 +0100
From: Marco Elver <elver@...gle.com>
To: andrey.konovalov@...ux.dev
Cc: Andrey Konovalov <andreyknvl@...il.com>,
Alexander Potapenko <glider@...gle.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Andrey Ryabinin <ryabinin.a.a@...il.com>,
kasan-dev@...glegroups.com, Peter Collingbourne <pcc@...gle.com>,
Evgenii Stepanov <eugenis@...gle.com>,
Florian Mayer <fmayer@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH] kasan: allow sampling page_alloc allocations for HW_TAGS
On Thu, Oct 27, 2022 at 10:10PM +0200, andrey.konovalov@...ux.dev wrote:
> From: Andrey Konovalov <andreyknvl@...gle.com>
>
> Add a new boot parameter called kasan.page_alloc.sample, which makes
> Hardware Tag-Based KASAN tag only every Nth page_alloc allocation.
>
> As Hardware Tag-Based KASAN is intended to be used in production, its
> performance impact is crucial. As page_alloc allocations tend to be big,
> tagging and checking all such allocations introduces a significant
> slowdown in some testing scenarios. The new flag allows to alleviate
> that slowdown.
>
> Enabling page_alloc sampling has a downside: KASAN will miss bad accesses
> to a page_alloc allocation that has not been tagged.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> ---
> Documentation/dev-tools/kasan.rst | 4 +++
> include/linux/kasan.h | 7 ++---
> mm/kasan/common.c | 9 +++++--
> mm/kasan/hw_tags.c | 26 +++++++++++++++++++
> mm/kasan/kasan.h | 15 +++++++++++
> mm/page_alloc.c | 43 +++++++++++++++++++++----------
> 6 files changed, 85 insertions(+), 19 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index 5c93ab915049..bd97301845ef 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -140,6 +140,10 @@ disabling KASAN altogether or controlling its features:
> - ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc
> allocations (default: ``on``).
>
> +- ``kasan.page_alloc.sample=<sampling frequency>`` makes KASAN tag only
Frequency is number of samples per frame (unit time, or if used
non-temporally like here, population size).
[1] https://en.wikipedia.org/wiki/Systematic_sampling
You're using it as an interval, so I'd just replace uses of frequency
with "interval" appropriately here and elsewhere.
> + every Nth page_alloc allocation, where N is the value of the parameter
> + (default: ``1``).
> +
> Error reports
> ~~~~~~~~~~~~~
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index d811b3d7d2a1..d45d45dfd007 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -120,12 +120,13 @@ static __always_inline void kasan_poison_pages(struct page *page,
> __kasan_poison_pages(page, order, init);
> }
>
> -void __kasan_unpoison_pages(struct page *page, unsigned int order, bool init);
> -static __always_inline void kasan_unpoison_pages(struct page *page,
> +bool __kasan_unpoison_pages(struct page *page, unsigned int order, bool init);
> +static __always_inline bool kasan_unpoison_pages(struct page *page,
> unsigned int order, bool init)
> {
> if (kasan_enabled())
> - __kasan_unpoison_pages(page, order, init);
> + return __kasan_unpoison_pages(page, order, init);
> + return false;
> }
>
> void __kasan_cache_create_kmalloc(struct kmem_cache *cache);
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 833bf2cfd2a3..1f30080a7a4c 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -95,19 +95,24 @@ asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
> }
> #endif /* CONFIG_KASAN_STACK */
>
> -void __kasan_unpoison_pages(struct page *page, unsigned int order, bool init)
> +bool __kasan_unpoison_pages(struct page *page, unsigned int order, bool init)
> {
> u8 tag;
> unsigned long i;
>
> if (unlikely(PageHighMem(page)))
> - return;
> + return false;
> +
> + if (!kasan_sample_page_alloc())
> + return false;
>
> tag = kasan_random_tag();
> kasan_unpoison(set_tag(page_address(page), tag),
> PAGE_SIZE << order, init);
> for (i = 0; i < (1 << order); i++)
> page_kasan_tag_set(page + i, tag);
> +
> + return true;
> }
>
> void __kasan_poison_pages(struct page *page, unsigned int order, bool init)
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index b22c4f461cb0..aa3b5a080297 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -59,6 +59,11 @@ EXPORT_SYMBOL_GPL(kasan_mode);
> /* Whether to enable vmalloc tagging. */
> DEFINE_STATIC_KEY_TRUE(kasan_flag_vmalloc);
>
> +/* Frequency of page_alloc allocation poisoning. */
> +unsigned long kasan_page_alloc_sample = 1;
> +
> +DEFINE_PER_CPU(unsigned long, kasan_page_alloc_count);
> +
> /* kasan=off/on */
> static int __init early_kasan_flag(char *arg)
> {
> @@ -122,6 +127,27 @@ static inline const char *kasan_mode_info(void)
> return "sync";
> }
>
> +/* kasan.page_alloc.sample=<sampling frequency> */
> +static int __init early_kasan_flag_page_alloc_sample(char *arg)
> +{
> + int rv;
> +
> + if (!arg)
> + return -EINVAL;
> +
> + rv = kstrtoul(arg, 0, &kasan_page_alloc_sample);
> + if (rv)
> + return rv;
> +
> + if (!kasan_page_alloc_sample) {
> + kasan_page_alloc_sample = 1;
> + return -EINVAL;
> + }
> +
> + return 0;
> +}
> +early_param("kasan.page_alloc.sample", early_kasan_flag_page_alloc_sample);
> +
> /*
> * kasan_init_hw_tags_cpu() is called for each CPU.
> * Not marked as __init as a CPU can be hot-plugged after boot.
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index abbcc1b0eec5..ee67eb35f4a7 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -42,6 +42,9 @@ enum kasan_mode {
>
> extern enum kasan_mode kasan_mode __ro_after_init;
>
> +extern unsigned long kasan_page_alloc_sample;
> +DECLARE_PER_CPU(unsigned long, kasan_page_alloc_count);
> +
> static inline bool kasan_vmalloc_enabled(void)
> {
> return static_branch_likely(&kasan_flag_vmalloc);
> @@ -57,6 +60,13 @@ static inline bool kasan_sync_fault_possible(void)
> return kasan_mode == KASAN_MODE_SYNC || kasan_mode == KASAN_MODE_ASYMM;
> }
>
> +static inline bool kasan_sample_page_alloc(void)
> +{
> + unsigned long *count = this_cpu_ptr(&kasan_page_alloc_count);
this_cpu_inc_return()
without it, you need to ensure preemption is disabled around here.
> +
> + return (*count)++ % kasan_page_alloc_sample == 0;
Doing '%' is a potentially costly operation if called in a fast-path.
We can generate better code with (rename 'count' -> 'skip'):
long skip_next = this_cpu_dec_return(kasan_page_alloc_skip);
if (skip_next < 0) {
this_cpu_write(kasan_page_alloc_skip, kasan_page_alloc_sample - 1);
return true;
}
return false;
Important is also to switch the counter to a 'long', otherwise you'd
have to pre-initialize all of them to something non-zero to avoid wrap.
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