cec.c

#include <linux/mm.h>
#include <linux/gfp.h>
#include <linux/kernel.h>

#include <asm/mce.h>

#include "debugfs.h"

/*
 * RAS Correctable Errors Collector
 *
 * This is a simple gadget which collects correctable errors and counts their
 * occurrence per physical page address.
 *
 * We've opted for possibly the simplest data structure to collect those - an
 * array of the size of a memory page. It stores 512 u64's with the following
 * structure:
 *
 * [63 ... PFN ... 12 | 11 ... generation ... 10 | 9 ... count ... 0]
 *
 * The generation in the two highest order bits is two bits which are set to 11b
 * on every insertion. During the course of each entry's existence, the
 * generation field gets decremented during spring cleaning to 10b, then 01b and
 * then 00b.
 *
 * This way we're employing the natural numeric ordering to make sure that newly
 * inserted/touched elements have higher 12-bit counts (which we've manufactured)
 * and thus iterating over the array initially won't kick out those elements
 * which were inserted last.
 *
 * Spring cleaning is what we do when we reach a certain number CLEAN_ELEMS of
 * elements entered into the array, during which, we're decaying all elements.
 * If, after decay, an element gets inserted again, its generation is set to 11b
 * to make sure it has higher numerical count than other, older elements and
 * thus emulate an an LRU-like behavior when deleting elements to free up space
 * in the page.
 *
 * When an element reaches it's max count of count_threshold, we try to poison
 * it by assuming that errors triggered count_threshold times in a single page
 * are excessive and that page shouldn't be used anymore. count_threshold is
 * initialized to COUNT_MASK which is the maximum.
 *
 * That error event entry causes cec_add_elem() to return !0 value and thus
 * signal to its callers to log the error.
 *
 * To the question why we've chosen a page and moving elements around with
 * memmove(), it is because it is a very simple structure to handle and max data
 * movement is 4K which on highly optimized modern CPUs is almost unnoticeable.
 * We wanted to avoid the pointer traversal of more complex structures like a
 * linked list or some sort of a balancing search tree.
 *
 * Deleting an element takes O(n) but since it is only a single page, it should
 * be fast enough and it shouldn't happen all too often depending on error
 * patterns.
 */

#undef pr_fmt
#define pr_fmt(fmt) "RAS: " fmt

/*
 * We use DECAY_BITS bits of PAGE_SHIFT bits for counting decay, i.e., how long
 * elements have stayed in the array without having been accessed again.
 */
#define DECAY_BITS		2
#define DECAY_MASK		((1ULL << DECAY_BITS) - 1)
#define MAX_ELEMS		(PAGE_SIZE / sizeof(u64))

/*
 * Threshold amount of inserted elements after which we start spring
 * cleaning.
 */
#define CLEAN_ELEMS		(MAX_ELEMS >> DECAY_BITS)

/* Bits which count the number of errors happened in this 4K page. */
#define COUNT_BITS		(PAGE_SHIFT - DECAY_BITS)
#define COUNT_MASK		((1ULL << COUNT_BITS) - 1)
#define FULL_COUNT_MASK		(PAGE_SIZE - 1)

/*
 * u64: [ 63 ... 12 | DECAY_BITS | COUNT_BITS ]
 */

#define PFN(e)			((e) >> PAGE_SHIFT)
#define DECAY(e)		(((e) >> COUNT_BITS) & DECAY_MASK)
#define COUNT(e)		((unsigned int)(e) & COUNT_MASK)
#define FULL_COUNT(e)		((e) & (PAGE_SIZE - 1))

static struct ce_array {
	u64 *array;			/* container page */
	unsigned int n;			/* number of elements in the array */

	unsigned int decay_count;	/*
					 * number of element insertions/increments
					 * since the last spring cleaning.
					 */

	u64 pfns_poisoned;		/*
					 * number of PFNs which got poisoned.
					 */

	u64 ces_entered;		/*
					 * The number of correctable errors
					 * entered into the collector.
					 */

	u64 decays_done;		/*
					 * Times we did spring cleaning.
					 */

	union {
		struct {
			__u32	disabled : 1,	/* cmdline disabled */
			__resv   : 31;
		};
		__u32 flags;
	};
} ce_arr;

static DEFINE_MUTEX(ce_mutex);
static u64 dfs_pfn;

/* Amount of errors after which we offline */
static unsigned int count_threshold = COUNT_MASK;

/*
 * The timer "decays" element count each timer_interval which is 24hrs by
 * default.
 */

#define CEC_TIMER_DEFAULT_INTERVAL	24 * 60 * 60	/* 24 hrs */
#define CEC_TIMER_MIN_INTERVAL		 1 * 60 * 60	/* 1h */
#define CEC_TIMER_MAX_INTERVAL	   30 *	24 * 60 * 60	/* one month */
static struct timer_list cec_timer;
static u64 timer_interval = CEC_TIMER_DEFAULT_INTERVAL;

/*
 * Decrement decay value. We're using DECAY_BITS bits to denote decay of an
 * element in the array. On insertion and any access, it gets reset to max.
 */
static void do_spring_cleaning(struct ce_array *ca)
{
	int i;

	for (i = 0; i < ca->n; i++) {
		u8 decay = DECAY(ca->array[i]);

		if (!decay)
			continue;

		decay--;

		ca->array[i] &= ~(DECAY_MASK << COUNT_BITS);
		ca->array[i] |= (decay << COUNT_BITS);
	}
	ca->decay_count = 0;
	ca->decays_done++;
}

/*
 * @interval in seconds
 */
static void cec_mod_timer(struct timer_list *t, unsigned long interval)
{
	unsigned long iv;

	iv = interval * HZ + jiffies;

	mod_timer(t, round_jiffies(iv));
}

static void cec_timer_fn(unsigned long data)
{
	struct ce_array *ca = (struct ce_array *)data;

	do_spring_cleaning(ca);

	cec_mod_timer(&cec_timer, timer_interval);
}

/*
 * @to: index of the smallest element which is >= then @pfn.
 *
 * Return the index of the pfn if found, otherwise negative value.
 */
static int __find_elem(struct ce_array *ca, u64 pfn, unsigned int *to)
{
	u64 this_pfn;
	int min = 0, max = ca->n;

	while (min < max) {
		int tmp = (max + min) >> 1;

		this_pfn = PFN(ca->array[tmp]);

		if (this_pfn < pfn)
			min = tmp + 1;
		else if (this_pfn > pfn)
			max = tmp;
		else {
			min = tmp;
			break;
		}
	}

	if (to)
		*to = min;

	this_pfn = PFN(ca->array[min]);

	if (this_pfn == pfn)
		return min;

	return -ENOKEY;
}

static int find_elem(struct ce_array *ca, u64 pfn, unsigned int *to)
{
	WARN_ON(!to);

	if (!ca->n) {
		*to = 0;
		return -ENOKEY;
	}
	return __find_elem(ca, pfn, to);
}

static void del_elem(struct ce_array *ca, int idx)
{
	/* Save us a function call when deleting the last element. */
	if (ca->n - (idx + 1))
		memmove((void *)&ca->array[idx],
			(void *)&ca->array[idx + 1],
			(ca->n - (idx + 1)) * sizeof(u64));

	ca->n--;
}

static u64 del_lru_elem_unlocked(struct ce_array *ca)
{
	unsigned int min = FULL_COUNT_MASK;
	int i, min_idx = 0;

	for (i = 0; i < ca->n; i++) {
		unsigned int this = FULL_COUNT(ca->array[i]);

		if (min > this) {
			min = this;
			min_idx = i;
		}
	}

	del_elem(ca, min_idx);

	return PFN(ca->array[min_idx]);
}

/*
 * We return the 0th pfn in the error case under the assumption that it cannot
 * be poisoned and excessive CEs in there are a serious deal anyway.
 */
static u64 __maybe_unused del_lru_elem(void)
{
	struct ce_array *ca = &ce_arr;
	u64 pfn;

	if (!ca->n)
		return 0;

	mutex_lock(&ce_mutex);
	pfn = del_lru_elem_unlocked(ca);
	mutex_unlock(&ce_mutex);

	return pfn;
}


int cec_add_elem(u64 pfn)
{
	struct ce_array *ca = &ce_arr;
	unsigned int to;
	int count, ret = 0;

	/*
	 * We can be called very early on the identify_cpu() path where we are
	 * not initialized yet. We ignore the error for simplicity.
	 */
	if (!ce_arr.array || ce_arr.disabled)
		return -ENODEV;

	ca->ces_entered++;

	mutex_lock(&ce_mutex);

	if (ca->n == MAX_ELEMS)
		WARN_ON(!del_lru_elem_unlocked(ca));

	ret = find_elem(ca, pfn, &to);
	if (ret < 0) {
		/*
		 * Shift range [to-end] to make room for one more element.
		 */
		memmove((void *)&ca->array[to + 1],
			(void *)&ca->array[to],
			(ca->n - to) * sizeof(u64));

		ca->array[to] = (pfn << PAGE_SHIFT) |
				(DECAY_MASK << COUNT_BITS) | 1;

		ca->n++;

		ret = 0;

		goto decay;
	}

	count = COUNT(ca->array[to]);

	if (count < count_threshold) {
		ca->array[to] |= (DECAY_MASK << COUNT_BITS);
		ca->array[to]++;

		ret = 0;
	} else {
		u64 pfn = ca->array[to] >> PAGE_SHIFT;

		if (!pfn_valid(pfn)) {
			pr_warn("CEC: Invalid pfn: 0x%llx\n", pfn);
		} else {
			/* We have reached max count for this page, soft-offline it. */
			pr_err("Soft-offlining pfn: 0x%llx\n", pfn);
			memory_failure_queue(pfn, 0, MF_SOFT_OFFLINE);
			ca->pfns_poisoned++;
		}

		del_elem(ca, to);

		/*
		 * Return a >0 value to denote that we've reached the offlining
		 * threshold.
		 */
		ret = 1;

		goto unlock;
	}

decay:
	ca->decay_count++;

	if (ca->decay_count >= CLEAN_ELEMS)
		do_spring_cleaning(ca);

unlock:
	mutex_unlock(&ce_mutex);

	return ret;
}

static int u64_get(void *data, u64 *val)
{
	*val = *(u64 *)data;

	return 0;
}

static int pfn_set(void *data, u64 val)
{
	*(u64 *)data = val;

	return cec_add_elem(val);
}

DEFINE_DEBUGFS_ATTRIBUTE(pfn_ops, u64_get, pfn_set, "0x%llx\n");

static int decay_interval_set(void *data, u64 val)
{
	*(u64 *)data = val;

	if (val < CEC_TIMER_MIN_INTERVAL)
		return -EINVAL;

	if (val > CEC_TIMER_MAX_INTERVAL)
		return -EINVAL;

	timer_interval = val;

	cec_mod_timer(&cec_timer, timer_interval);
	return 0;
}
DEFINE_DEBUGFS_ATTRIBUTE(decay_interval_ops, u64_get, decay_interval_set, "%lld\n");

static int count_threshold_set(void *data, u64 val)
{
	*(u64 *)data = val;

	if (val > COUNT_MASK)
		val = COUNT_MASK;

	count_threshold = val;

	return 0;
}
DEFINE_DEBUGFS_ATTRIBUTE(count_threshold_ops, u64_get, count_threshold_set, "%lld\n");

static int array_dump(struct seq_file *m, void *v)
{
	struct ce_array *ca = &ce_arr;
	u64 prev = 0;
	int i;

	mutex_lock(&ce_mutex);

	seq_printf(m, "{ n: %d\n", ca->n);
	for (i = 0; i < ca->n; i++) {
		u64 this = PFN(ca->array[i]);

		seq_printf(m, " %03d: [%016llx|%03llx]\n", i, this, FULL_COUNT(ca->array[i]));

		WARN_ON(prev > this);

		prev = this;
	}

	seq_printf(m, "}\n");

	seq_printf(m, "Stats:\nCEs: %llu\nofflined pages: %llu\n",
		   ca->ces_entered, ca->pfns_poisoned);

	seq_printf(m, "Flags: 0x%x\n", ca->flags);

	seq_printf(m, "Timer interval: %lld seconds\n", timer_interval);
	seq_printf(m, "Decays: %lld\n", ca->decays_done);

	seq_printf(m, "Action threshold: %d\n", count_threshold);

	mutex_unlock(&ce_mutex);

	return 0;
}

static int array_open(struct inode *inode, struct file *filp)
{
	return single_open(filp, array_dump, NULL);
}

static const struct file_operations array_ops = {
	.owner	 = THIS_MODULE,
	.open	 = array_open,
	.read	 = seq_read,
	.llseek	 = seq_lseek,
	.release = single_release,
};

static int __init create_debugfs_nodes(void)
{
	struct dentry *d, *pfn, *decay, *count, *array;

	d = debugfs_create_dir("cec", ras_debugfs_dir);
	if (!d) {
		pr_warn("Error creating cec debugfs node!\n");
		return -1;
	}

	pfn = debugfs_create_file("pfn", S_IRUSR | S_IWUSR, d, &dfs_pfn, &pfn_ops);
	if (!pfn) {
		pr_warn("Error creating pfn debugfs node!\n");
		goto err;
	}

	array = debugfs_create_file("array", S_IRUSR, d, NULL, &array_ops);
	if (!array) {
		pr_warn("Error creating array debugfs node!\n");
		goto err;
	}

	decay = debugfs_create_file("decay_interval", S_IRUSR | S_IWUSR, d,
				    &timer_interval, &decay_interval_ops);
	if (!decay) {
		pr_warn("Error creating decay_interval debugfs node!\n");
		goto err;
	}

	count = debugfs_create_file("count_threshold", S_IRUSR | S_IWUSR, d,
				    &count_threshold, &count_threshold_ops);
	if (!decay) {
		pr_warn("Error creating count_threshold debugfs node!\n");
		goto err;
	}


	return 0;

err:
	debugfs_remove_recursive(d);

	return 1;
}

void __init cec_init(void)
{
	if (ce_arr.disabled)
		return;

	ce_arr.array = (void *)get_zeroed_page(GFP_KERNEL);
	if (!ce_arr.array) {
		pr_err("Error allocating CE array page!\n");
		return;
	}

	if (create_debugfs_nodes())
		return;

	setup_timer(&cec_timer, cec_timer_fn, (unsigned long)&ce_arr);
	cec_mod_timer(&cec_timer, CEC_TIMER_DEFAULT_INTERVAL);

	pr_info("Correctable Errors collector initialized.\n");
}

int __init parse_cec_param(char *str)
{
	if (!str)
		return 0;

	if (*str == '=')
		str++;

	if (!strncmp(str, "cec_disable", 7))
		ce_arr.disabled = 1;
	else
		return 0;

	return 1;
}