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Date: Tue, 26 May 2009 19:37:36 +0200
From: Johannes Weiner <hannes@...xchg.org>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
"hugh.dickins@...cali.co.uk" <hugh.dickins@...cali.co.uk>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 3/5] count cache-only swaps
On Tue, May 26, 2009 at 12:16:38PM +0900, KAMEZAWA Hiroyuki wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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@...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?
Hannes
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