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SubjectRe: [PATCH v3] kasan: allow sampling page_alloc allocations for HW_TAGS
On Fri, 16 Dec 2022 at 23:17, <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@google.com>
>
> 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 can introduce a significant
> slowdown.
>
> Add two new boot parameters that allow to alleviate that slowdown:
>
> - kasan.page_alloc.sample, which makes Hardware Tag-Based KASAN tag only
> every Nth page_alloc allocation with the order configured by the second
> added parameter (default: tag every such allocation).
>
> - kasan.page_alloc.sample.order, which makes sampling enabled by the first
> parameter only affect page_alloc allocations with the order equal or
> greater than the specified value (default: 3, see below).
>
> The exact performance improvement caused by using the new parameters
> depends on their values and the applied workload.
>
> The chosen default value for kasan.page_alloc.sample.order is 3, which
> matches both PAGE_ALLOC_COSTLY_ORDER and SKB_FRAG_PAGE_ORDER. This is done
> for two reasons:
>
> 1. PAGE_ALLOC_COSTLY_ORDER is "the order at which allocations are deemed
> costly to service", which corresponds to the idea that only large and
> thus costly allocations are supposed to sampled.
>
> 2. One of the workloads targeted by this patch is a benchmark that sends
> a large amount of data over a local loopback connection. Most multi-page
> data allocations in the networking subsystem have the order of
> SKB_FRAG_PAGE_ORDER (or PAGE_ALLOC_COSTLY_ORDER).
>
> When running a local loopback test on a testing MTE-enabled device in sync
> mode, enabling Hardware Tag-Based KASAN introduces a ~50% slowdown.
> Applying this patch and setting kasan.page_alloc.sampling to a value higher
> than 1 allows to lower the slowdown. The performance improvement saturates
> around the sampling interval value of 10 with the default sampling page
> order of 3. This lowers the slowdown to ~20%. The slowdown in real
> scenarios involving the network will likely be better.
>
> Enabling page_alloc sampling has a downside: KASAN misses bad accesses to
> a page_alloc allocation that has not been tagged. This lowers the value of
> KASAN as a security mitigation.
>
> However, based on measuring the number of page_alloc allocations of
> different orders during boot in a test build, sampling with the default
> kasan.page_alloc.sample.order value affects only ~7% of allocations.
> The rest ~93% of allocations are still checked deterministically.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

On a whole:

Reviewed-by: Marco Elver <elver@google.com>

This looks much better, given it'll automatically do the right thing
without marking costly allocation sites.

Minor comments below.

> ---
>
> Changes v2-v3:
> - Drop __GFP_KASAN_SAMPLE flag.
> - Add kasan.page_alloc.sample.order.
> - Add fast-path for disabled sampling to kasan_sample_page_alloc.
>
> Changes v1->v2:
> - Only sample allocations when __GFP_KASAN_SAMPLE is provided to
> alloc_pages().
> - Fix build when KASAN is disabled.
> - Add more information about the flag to documentation.
> - Use optimized preemption-safe approach for sampling suggested by Marco.
> ---
> Documentation/dev-tools/kasan.rst | 16 +++++++++
> include/linux/kasan.h | 14 +++++---
> mm/kasan/common.c | 9 +++--
> mm/kasan/hw_tags.c | 60 +++++++++++++++++++++++++++++++
> mm/kasan/kasan.h | 27 ++++++++++++++
> mm/page_alloc.c | 43 ++++++++++++++--------
> 6 files changed, 148 insertions(+), 21 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index 5c93ab915049..d983e4fcee7c 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -140,6 +140,22 @@ 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 interval>`` makes KASAN tag only every
> + Nth page_alloc allocation with the order equal or greater than
> + ``kasan.page_alloc.sample.order``, where N is the value of the ``sample``
> + parameter (default: ``1``, or tag every such allocation).
> + This parameter is intended to mitigate the performance overhead introduced
> + by KASAN.
> + Note that enabling this parameter makes Hardware Tag-Based KASAN skip checks
> + of allocations chosen by sampling and thus miss bad accesses to these
> + allocations. Use the default value for accurate bug detection.
> +
> +- ``kasan.page_alloc.sample.order=<minimum page order>`` specifies the minimum
> + order of allocations that are affected by sampling (default: ``3``).
> + Only applies when ``kasan.page_alloc.sample`` is set to a non-default value.

"set to a value greater than 1"? The additional indirection through
"non-default" seems unnecessary.

> + This parameter is intended to allow sampling only large page_alloc
> + allocations, which is the biggest source of the performace overhead.

s/performace/performance/

> +
> Error reports
> ~~~~~~~~~~~~~
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index 96c9d56e5510..5ebbaf672009 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);
> @@ -249,8 +250,11 @@ static __always_inline bool kasan_check_byte(const void *addr)
> static inline void kasan_unpoison_range(const void *address, size_t size) {}
> static inline void kasan_poison_pages(struct page *page, unsigned int order,
> bool init) {}
> -static inline void kasan_unpoison_pages(struct page *page, unsigned int order,
> - bool init) {}
> +static inline bool kasan_unpoison_pages(struct page *page, unsigned int order,
> + bool init)
> +{
> + return false;
> +}
> static inline void kasan_cache_create_kmalloc(struct kmem_cache *cache) {}
> static inline void kasan_poison_slab(struct slab *slab) {}
> static inline void kasan_unpoison_object_data(struct kmem_cache *cache,
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 833bf2cfd2a3..1d0008e1c420 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(order))
> + 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..d1bcb0205327 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -59,6 +59,24 @@ EXPORT_SYMBOL_GPL(kasan_mode);
> /* Whether to enable vmalloc tagging. */
> DEFINE_STATIC_KEY_TRUE(kasan_flag_vmalloc);
>
> +#define PAGE_ALLOC_SAMPLE_DEFAULT 1
> +#define PAGE_ALLOC_SAMPLE_ORDER_DEFAULT 3

Why not just set it to PAGE_ALLOC_COSTLY_ORDER?

> +/*
> + * Sampling interval of page_alloc allocation (un)poisoning.
> + * Defaults to no sampling.
> + */
> +unsigned long kasan_page_alloc_sample = PAGE_ALLOC_SAMPLE_DEFAULT;
> +
> +/*
> + * Minimum order of page_alloc allocations to be affected by sampling.
> + * The default value is chosen to match both
> + * PAGE_ALLOC_COSTLY_ORDER and SKB_FRAG_PAGE_ORDER.
> + */
> +unsigned int kasan_page_alloc_sample_order = PAGE_ALLOC_SAMPLE_ORDER_DEFAULT;
> +
> +DEFINE_PER_CPU(long, kasan_page_alloc_skip);
> +
> /* kasan=off/on */
> static int __init early_kasan_flag(char *arg)
> {
> @@ -122,6 +140,48 @@ static inline const char *kasan_mode_info(void)
> return "sync";
> }
>
> +/* kasan.page_alloc.sample=<sampling interval> */
> +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 > LONG_MAX) {
> + kasan_page_alloc_sample = PAGE_ALLOC_SAMPLE_DEFAULT;
> + return -EINVAL;
> + }
> +
> + return 0;
> +}
> +early_param("kasan.page_alloc.sample", early_kasan_flag_page_alloc_sample);
> +
> +/* kasan.page_alloc.sample.order=<minimum page order> */
> +static int __init early_kasan_flag_page_alloc_sample_order(char *arg)
> +{
> + int rv;
> +
> + if (!arg)
> + return -EINVAL;
> +
> + rv = kstrtouint(arg, 0, &kasan_page_alloc_sample_order);
> + if (rv)
> + return rv;
> +
> + if (kasan_page_alloc_sample_order > INT_MAX) {
> + kasan_page_alloc_sample_order = PAGE_ALLOC_SAMPLE_ORDER_DEFAULT;
> + return -EINVAL;
> + }
> +
> + return 0;
> +}
> +early_param("kasan.page_alloc.sample.order", early_kasan_flag_page_alloc_sample_order);
> +
> /*
> * 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 ea8cf1310b1e..32413f22aa82 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -42,6 +42,10 @@ enum kasan_mode {
>
> extern enum kasan_mode kasan_mode __ro_after_init;
>
> +extern unsigned long kasan_page_alloc_sample;
> +extern unsigned int kasan_page_alloc_sample_order;
> +DECLARE_PER_CPU(long, kasan_page_alloc_skip);
> +
> static inline bool kasan_vmalloc_enabled(void)
> {
> return static_branch_likely(&kasan_flag_vmalloc);
> @@ -57,6 +61,24 @@ 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(unsigned int order)
> +{
> + /* Fast-path for when sampling is disabled. */
> + if (kasan_page_alloc_sample == 1)
> + return true;
> +
> + if (order < kasan_page_alloc_sample_order)
> + return true;
> +
> + if (this_cpu_dec_return(kasan_page_alloc_skip) < 0) {
> + this_cpu_write(kasan_page_alloc_skip,
> + kasan_page_alloc_sample - 1);
> + return true;
> + }
> +
> + return false;
> +}
> +
> #else /* CONFIG_KASAN_HW_TAGS */
>
> static inline bool kasan_async_fault_possible(void)
> @@ -69,6 +91,11 @@ static inline bool kasan_sync_fault_possible(void)
> return true;
> }
>
> +static inline bool kasan_sample_page_alloc(unsigned int order)
> +{
> + return true;
> +}
> +
> #endif /* CONFIG_KASAN_HW_TAGS */
>
> #ifdef CONFIG_KASAN_GENERIC
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0745aedebb37..7d980dc0000e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1356,6 +1356,8 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
> * see the comment next to it.
> * 3. Skipping poisoning is requested via __GFP_SKIP_KASAN_POISON,
> * see the comment next to it.
> + * 4. The allocation is excluded from being checked due to sampling,
> + * see the call to kasan_unpoison_pages.
> *
> * Poisoning pages during deferred memory init will greatly lengthen the
> * process and cause problem in large memory systems as the deferred pages
> @@ -2468,7 +2470,8 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
> {
> bool init = !want_init_on_free() && want_init_on_alloc(gfp_flags) &&
> !should_skip_init(gfp_flags);
> - bool init_tags = init && (gfp_flags & __GFP_ZEROTAGS);
> + bool zero_tags = init && (gfp_flags & __GFP_ZEROTAGS);
> + bool reset_tags = !zero_tags;
> int i;
>
> set_page_private(page, 0);
> @@ -2491,30 +2494,42 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
> */
>
> /*
> - * If memory tags should be zeroed (which happens only when memory
> - * should be initialized as well).
> + * If memory tags should be zeroed
> + * (which happens only when memory should be initialized as well).
> */
> - if (init_tags) {
> + if (zero_tags) {
> /* Initialize both memory and tags. */
> for (i = 0; i != 1 << order; ++i)
> tag_clear_highpage(page + i);
>
> - /* Note that memory is already initialized by the loop above. */
> + /* Take note that memory was initialized by the loop above. */
> init = false;
> }
> if (!should_skip_kasan_unpoison(gfp_flags)) {
> - /* Unpoison shadow memory or set memory tags. */
> - kasan_unpoison_pages(page, order, init);
> -
> - /* Note that memory is already initialized by KASAN. */
> - if (kasan_has_integrated_init())
> - init = false;
> - } else {
> - /* Ensure page_address() dereferencing does not fault. */
> + /* Try unpoisoning (or setting tags) and initializing memory. */
> + if (kasan_unpoison_pages(page, order, init)) {
> + /* Take note that memory was initialized by KASAN. */
> + if (kasan_has_integrated_init())
> + init = false;
> + /* Take note that memory tags were set by KASAN. */
> + reset_tags = false;
> + } else {
> + /*
> + * KASAN decided to exclude this allocation from being
> + * poisoned due to sampling. Skip poisoning as well.
> + */
> + SetPageSkipKASanPoison(page);
> + }
> + }
> + /*
> + * If memory tags have not been set, reset the page tags to ensure
> + * page_address() dereferencing does not fault.
> + */
> + if (reset_tags) {
> for (i = 0; i != 1 << order; ++i)
> page_kasan_tag_reset(page + i);
> }
> - /* If memory is still not initialized, do it now. */
> + /* If memory is still not initialized, initialize it now. */
> if (init)
> kernel_init_pages(page, 1 << order);
> /* Propagate __GFP_SKIP_KASAN_POISON to page flags. */
> --
> 2.25.1
>

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