/*
 * Lockless hierarchical page accounting & limiting
 *
 * Copyright (C) 2014 Red Hat, Inc., Johannes Weiner
 */

#include <linux/page_counter.h>
#include <linux/atomic.h>
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/sched.h>
#include <linux/bug.h>
#include <asm/page.h>

/**
 * page_counter_cancel - take pages out of the local counter
 * @counter: counter
 * @nr_pages: number of pages to cancel
 */
void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages)
{
        long new;

        new = atomic_long_sub_return(nr_pages, &counter->count);
        /* More uncharges than charges? */
        WARN_ON_ONCE(new < 0);
}

/**
 * page_counter_charge - hierarchically charge pages
 * @counter: counter
 * @nr_pages: number of pages to charge
 *
 * NOTE: This does not consider any configured counter limits.
 */
void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
{
        struct page_counter *c;

        for (c = counter; c; c = c->parent) {
                long new;

                new = atomic_long_add_return(nr_pages, &c->count);
                /*
                 * This is indeed racy, but we can live with some
                 * inaccuracy in the watermark.
                 */
                if (new > c->watermark)
                        c->watermark = new;
        }
}

/**
 * page_counter_try_charge - try to hierarchically charge pages
 * @counter: counter
 * @nr_pages: number of pages to charge
 * @fail: points first counter to hit its limit, if any
 *
 * Returns %true on success, or %false and @fail if the counter or one
 * of its ancestors has hit its configured limit.
 */
bool page_counter_try_charge(struct page_counter *counter,
                             unsigned long nr_pages,
                             struct page_counter **fail)
{
        struct page_counter *c;

        for (c = counter; c; c = c->parent) {
                long new;
                /*
                 * Charge speculatively to avoid an expensive CAS.  If
                 * a bigger charge fails, it might falsely lock out a
                 * racing smaller charge and send it into reclaim
                 * early, but the error is limited to the difference
                 * between the two sizes, which is less than 2M/4M in
                 * case of a THP locking out a regular page charge.
                 *
                 * The atomic_long_add_return() implies a full memory
                 * barrier between incrementing the count and reading
                 * the limit.  When racing with page_counter_limit(),
                 * we either see the new limit or the setter sees the
                 * counter has changed and retries.
                 */
                new = atomic_long_add_return(nr_pages, &c->count);
                if (new > c->limit) {
                        atomic_long_sub(nr_pages, &c->count);
                        /*
                         * This is racy, but we can live with some
                         * inaccuracy in the failcnt.
                         */
                        c->failcnt++;
                        *fail = c;
                        goto failed;
                }
                /*
                 * Just like with failcnt, we can live with some
                 * inaccuracy in the watermark.
                 */
                if (new > c->watermark)
                        c->watermark = new;
        }
        return true;

failed:
        for (c = counter; c != *fail; c = c->parent)
                page_counter_cancel(c, nr_pages);

        return false;
}

/**
 * page_counter_uncharge - hierarchically uncharge pages
 * @counter: counter
 * @nr_pages: number of pages to uncharge
 */
void page_counter_uncharge(struct page_counter *counter, unsigned long nr_pages)
{
        struct page_counter *c;

        for (c = counter; c; c = c->parent)
                page_counter_cancel(c, nr_pages);
}

/**
 * page_counter_limit - limit the number of pages allowed
 * @counter: counter
 * @limit: limit to set
 *
 * Returns 0 on success, -EBUSY if the current number of pages on the
 * counter already exceeds the specified limit.
 *
 * The caller must serialize invocations on the same counter.
 */
int page_counter_limit(struct page_counter *counter, unsigned long limit)
{
        for (;;) {
                unsigned long old;
                long count;

                /*
                 * Update the limit while making sure that it's not
                 * below the concurrently-changing counter value.
                 *
                 * The xchg implies two full memory barriers before
                 * and after, so the read-swap-read is ordered and
                 * ensures coherency with page_counter_try_charge():
                 * that function modifies the count before checking
                 * the limit, so if it sees the old limit, we see the
                 * modified counter and retry.
                 */
                count = atomic_long_read(&counter->count);

                if (count > limit)
                        return -EBUSY;

                old = xchg(&counter->limit, limit);

                if (atomic_long_read(&counter->count) <= count)
                        return 0;

                counter->limit = old;
                cond_resched();
        }
}

/**
 * page_counter_memparse - memparse() for page counter limits
 * @buf: string to parse
 * @max: string meaning maximum possible value
 * @nr_pages: returns the result in number of pages
 *
 * Returns -EINVAL, or 0 and @nr_pages on success.  @nr_pages will be
 * limited to %PAGE_COUNTER_MAX.
 */
int page_counter_memparse(const char *buf, const char *max,
                          unsigned long *nr_pages)
{
        char *end;
        u64 bytes;

        if (!strcmp(buf, max)) {
                *nr_pages = PAGE_COUNTER_MAX;
                return 0;
        }

        bytes = memparse(buf, &end);
        if (*end != '\0')
                return -EINVAL;

        *nr_pages = min(bytes / PAGE_SIZE, (u64)PAGE_COUNTER_MAX);

        return 0;
}