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    SubjectRe: [RFC][PATCH 3/5] count cache-only swaps
    On Tue, 26 May 2009 19:37:36 +0200
    Johannes Weiner <hannes@cmpxchg.org> wrote:

    > On Tue, May 26, 2009 at 12:16:38PM +0900, KAMEZAWA Hiroyuki wrote:
    > > 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));
    > > }
    >
    > This is shown rather seldomly (sysrq and oom), for that purpose two
    > counters are overkill. Maybe remove the global one and sum up the
    > per-swapdevice counters on demand?
    >
    One for scanning (as 5/5), One for checking we should scan or not.
    But yes, I feel 2 counters may be overkilling....
    Maybe I'll remove global counter in the next version.

    Thanks,
    -Kame






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