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    Subject[RFC][PATCH 3/5] count cache-only swaps
    From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

    This patch adds a counter for unused swap caches.
    Maybe useful to see "we're really under shortage of swap".

    The value can be seen as kernel message at Sysrq-m etc.

    Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    ---
    include/linux/swap.h | 3 +++
    mm/swap_state.c | 2 ++
    mm/swapfile.c | 23 ++++++++++++++++++++---
    3 files changed, 25 insertions(+), 3 deletions(-)
    Index: new-trial-swapcount/include/linux/swap.h
    ===================================================================
    --- new-trial-swapcount.orig/include/linux/swap.h
    +++ new-trial-swapcount/include/linux/swap.h
    @@ -155,6 +155,7 @@ struct swap_info_struct {
    unsigned int max;
    unsigned int inuse_pages;
    unsigned int old_block_size;
    + unsigned int cache_only;
    };

    struct swap_list_t {
    @@ -298,6 +299,7 @@ extern struct page *swapin_readahead(swp
    /* linux/mm/swapfile.c */
    extern long nr_swap_pages;
    extern long total_swap_pages;
    +extern long nr_cache_only_swaps;
    extern void si_swapinfo(struct sysinfo *);
    extern swp_entry_t get_swap_page(void);
    extern swp_entry_t get_swap_page_of_type(int);
    @@ -358,6 +360,7 @@ static inline void mem_cgroup_uncharge_s
    #define nr_swap_pages 0L
    #define total_swap_pages 0L
    #define total_swapcache_pages 0UL
    +#define nr_cache_only_swaps 0UL

    #define si_swapinfo(val) \
    do { (val)->freeswap = (val)->totalswap = 0; } while (0)
    Index: new-trial-swapcount/mm/swapfile.c
    ===================================================================
    --- new-trial-swapcount.orig/mm/swapfile.c
    +++ new-trial-swapcount/mm/swapfile.c
    @@ -39,6 +39,7 @@ static DEFINE_SPINLOCK(swap_lock);
    static unsigned int nr_swapfiles;
    long nr_swap_pages;
    long total_swap_pages;
    +long nr_cache_only_swaps;
    static int swap_overflow;
    static int least_priority;

    @@ -306,9 +307,11 @@ checks:
    si->lowest_bit = si->max;
    si->highest_bit = 0;
    }
    - if (cache) /* at usual swap-out via vmscan.c */
    + if (cache) {/* at usual swap-out via vmscan.c */
    si->swap_map[offset] = make_swap_count(0, 1);
    - else /* at suspend */
    + si->cache_only++;
    + nr_cache_only_swaps++;
    + } else /* at suspend */
    si->swap_map[offset] = make_swap_count(1, 0);
    si->cluster_next = offset + 1;
    si->flags -= SWP_SCANNING;
    @@ -513,7 +516,10 @@ static int swap_entry_free(struct swap_i
    } else { /* dropping swap cache flag */
    VM_BUG_ON(!has_cache);
    p->swap_map[offset] = make_swap_count(count, 0);
    -
    + if (!count) {
    + p->cache_only--;
    + nr_cache_only_swaps--;
    + }
    }
    /* return code. */
    count = p->swap_map[offset];
    @@ -529,6 +535,11 @@ static int swap_entry_free(struct swap_i
    p->inuse_pages--;
    mem_cgroup_uncharge_swap(ent);
    }
    + if (swap_has_cache(count) && !swap_count(count)) {
    + nr_cache_only_swaps++;
    + p->cache_only++;
    + }
    +
    return count;
    }

    @@ -1128,6 +1139,8 @@ static int try_to_unuse(unsigned int typ
    if (swap_count(*swap_map) == SWAP_MAP_MAX) {
    spin_lock(&swap_lock);
    *swap_map = make_swap_count(0, 1);
    + si->cache_only++;
    + nr_cache_only_swaps++;
    spin_unlock(&swap_lock);
    reset_overflow = 1;
    }
    @@ -2033,6 +2046,10 @@ static int __swap_duplicate(swp_entry_t
    if (count < SWAP_MAP_MAX - 1) {
    p->swap_map[offset] = make_swap_count(count + 1,
    has_cache);
    + if (has_cache && !count) {
    + p->cache_only--;
    + nr_cache_only_swaps--;
    + }
    result = 1;
    } else if (count <= SWAP_MAP_MAX) {
    if (swap_overflow++ < 5)
    Index: new-trial-swapcount/mm/swap_state.c
    ===================================================================
    --- new-trial-swapcount.orig/mm/swap_state.c
    +++ new-trial-swapcount/mm/swap_state.c
    @@ -63,6 +63,8 @@ void show_swap_cache_info(void)
    swap_cache_info.find_success, swap_cache_info.find_total);
    printk("Free swap = %ldkB\n", nr_swap_pages << (PAGE_SHIFT - 10));
    printk("Total swap = %lukB\n", total_swap_pages << (PAGE_SHIFT - 10));
    + printk("Cache only swap = %lukB\n",
    + nr_cache_only_swaps << (PAGE_SHIFT - 10));
    }

    /*


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