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SubjectRe: [PATCH 6/9] swap_info: swap_map of chars not shorts
On Thu, 15 Oct 2009 01:53:52 +0100 (BST)
Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote:

> Halve the vmalloc'ed swap_map array from unsigned shorts to unsigned
> chars: it's still very unusual to reach a swap count of 126, and the
> next patch allows it to be extended indefinitely.
>
> Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

> ---
>
> include/linux/swap.h | 8 ++++----
> mm/swapfile.c | 40 +++++++++++++++++++++++-----------------
> 2 files changed, 27 insertions(+), 21 deletions(-)
>
> --- si5/include/linux/swap.h 2009-10-14 21:26:42.000000000 +0100
> +++ si6/include/linux/swap.h 2009-10-14 21:26:49.000000000 +0100
> @@ -151,9 +151,9 @@ enum {
>
> #define SWAP_CLUSTER_MAX 32
>
> -#define SWAP_MAP_MAX 0x7ffe
> -#define SWAP_MAP_BAD 0x7fff
> -#define SWAP_HAS_CACHE 0x8000 /* There is a swap cache of entry. */
> +#define SWAP_MAP_MAX 0x7e
> +#define SWAP_MAP_BAD 0x7f
> +#define SWAP_HAS_CACHE 0x80 /* There is a swap cache of entry. */
>
> /*
> * The in-memory structure used to track swap areas.
> @@ -167,7 +167,7 @@ struct swap_info_struct {
> struct block_device *bdev;
> struct swap_extent first_swap_extent;
> struct swap_extent *curr_swap_extent;
> - unsigned short *swap_map;
> + unsigned char *swap_map;
> unsigned int lowest_bit;
> unsigned int highest_bit;
> unsigned int lowest_alloc; /* while preparing discard cluster */
> --- si5/mm/swapfile.c 2009-10-14 21:26:42.000000000 +0100
> +++ si6/mm/swapfile.c 2009-10-14 21:26:49.000000000 +0100
> @@ -53,7 +53,7 @@ static struct swap_info_struct *swap_inf
>
> static DEFINE_MUTEX(swapon_mutex);
>
> -static inline int swap_count(unsigned short ent)
> +static inline unsigned char swap_count(unsigned char ent)
> {
> return ent & ~SWAP_HAS_CACHE;
> }
> @@ -203,7 +203,7 @@ static int wait_for_discard(void *word)
> #define LATENCY_LIMIT 256
>
> static inline unsigned long scan_swap_map(struct swap_info_struct *si,
> - unsigned short usage)
> + unsigned char usage)
> {
> unsigned long offset;
> unsigned long scan_base;
> @@ -531,12 +531,12 @@ out:
> return NULL;
> }
>
> -static unsigned short swap_entry_free(struct swap_info_struct *p,
> - swp_entry_t entry, unsigned short usage)
> +static unsigned char swap_entry_free(struct swap_info_struct *p,
> + swp_entry_t entry, unsigned char usage)
> {
> unsigned long offset = swp_offset(entry);
> - unsigned short count;
> - unsigned short has_cache;
> + unsigned char count;
> + unsigned char has_cache;
>
> count = p->swap_map[offset];
> has_cache = count & SWAP_HAS_CACHE;
> @@ -591,7 +591,7 @@ void swap_free(swp_entry_t entry)
> void swapcache_free(swp_entry_t entry, struct page *page)
> {
> struct swap_info_struct *p;
> - unsigned short count;
> + unsigned char count;
>
> p = swap_info_get(entry);
> if (p) {
> @@ -975,7 +975,7 @@ static unsigned int find_next_to_unuse(s
> {
> unsigned int max = si->max;
> unsigned int i = prev;
> - int count;
> + unsigned char count;
>
> /*
> * No need for swap_lock here: we're just looking
> @@ -1013,8 +1013,8 @@ static int try_to_unuse(unsigned int typ
> {
> struct swap_info_struct *si = swap_info[type];
> struct mm_struct *start_mm;
> - unsigned short *swap_map;
> - unsigned short swcount;
> + unsigned char *swap_map;
> + unsigned char swcount;
> struct page *page;
> swp_entry_t entry;
> unsigned int i = 0;
> @@ -1175,6 +1175,12 @@ static int try_to_unuse(unsigned int typ
> * If that's wrong, then we should worry more about
> * exit_mmap() and do_munmap() cases described above:
> * we might be resetting SWAP_MAP_MAX too early here.
> + *
> + * Yes, that's wrong: though very unlikely, swap count 0x7ffe
> + * could surely occur if pid_max raised from PID_MAX_DEFAULT;

Just a nitpick.

Hmm, logically, our MAX COUNT is 0x7e after this patch. Then, how about not
mentioning to 0x7ffe and PID_MAX ? as..

Yes, that's wrong: we now use SWAP_MAP_MAX as 0x7e, very easy to overflow.
next patch will...


Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

> + * and we are now lowering SWAP_MAP_MAX to 0x7e, making it
> + * much easier to reach. But the next patch will fix that.
> + *
> * We know "Undead"s can happen, they're okay, so don't
> * report them; but do report if we reset SWAP_MAP_MAX.
> */
> @@ -1494,7 +1500,7 @@ bad_bmap:
> SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> {
> struct swap_info_struct *p = NULL;
> - unsigned short *swap_map;
> + unsigned char *swap_map;
> struct file *swap_file, *victim;
> struct address_space *mapping;
> struct inode *inode;
> @@ -1764,7 +1770,7 @@ SYSCALL_DEFINE2(swapon, const char __use
> sector_t span;
> unsigned long maxpages = 1;
> unsigned long swapfilepages;
> - unsigned short *swap_map = NULL;
> + unsigned char *swap_map = NULL;
> struct page *page = NULL;
> struct inode *inode = NULL;
> int did_down = 0;
> @@ -1939,13 +1945,13 @@ SYSCALL_DEFINE2(swapon, const char __use
> goto bad_swap;
>
> /* OK, set up the swap map and apply the bad block list */
> - swap_map = vmalloc(maxpages * sizeof(short));
> + swap_map = vmalloc(maxpages);
> if (!swap_map) {
> error = -ENOMEM;
> goto bad_swap;
> }
>
> - memset(swap_map, 0, maxpages * sizeof(short));
> + memset(swap_map, 0, maxpages);
> for (i = 0; i < swap_header->info.nr_badpages; i++) {
> int page_nr = swap_header->info.badpages[i];
> if (page_nr <= 0 || page_nr >= swap_header->info.last_page) {
> @@ -2083,12 +2089,12 @@ void si_swapinfo(struct sysinfo *val)
> * - swap-cache reference is requested but there is already one. -> EEXIST
> * - swap-cache reference is requested but the entry is not used. -> ENOENT
> */
> -static int __swap_duplicate(swp_entry_t entry, unsigned short usage)
> +static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
> {
> struct swap_info_struct *p;
> unsigned long offset, type;
> - unsigned short count;
> - unsigned short has_cache;
> + unsigned char count;
> + unsigned char has_cache;
> int err = -EINVAL;
>
> if (non_swap_entry(entry))
>



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