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SubjectRe: [PATCH 8/9] swap_info: note SWAP_MAP_SHMEM
On Thu, 15 Oct 2009 01:57:28 +0100 (BST)
Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote:

> While we're fiddling with the swap_map values, let's assign a particular
> value to shmem/tmpfs swap pages: their swap counts are never incremented,
> and it helps swapoff's try_to_unuse() a little if it can immediately
> distinguish those pages from process pages.
>
> Since we've no use for SWAP_MAP_BAD | COUNT_CONTINUED,
> we might as well use that 0xbf value for SWAP_MAP_SHMEM.
>
> Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

I welcome this!

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

> ---
>
> include/linux/swap.h | 6 +++++
> mm/shmem.c | 11 +++++++--
> mm/swapfile.c | 47 +++++++++++++++++++++++------------------
> 3 files changed, 42 insertions(+), 22 deletions(-)
>
> --- si7/include/linux/swap.h 2009-10-14 21:26:57.000000000 +0100
> +++ si8/include/linux/swap.h 2009-10-14 21:27:07.000000000 +0100
> @@ -157,6 +157,7 @@ enum {
> #define SWAP_HAS_CACHE 0x40 /* Flag page is cached, in first swap_map */
> #define SWAP_CONT_MAX 0x7f /* Max count, in each swap_map continuation */
> #define COUNT_CONTINUED 0x80 /* See swap_map continuation for full count */
> +#define SWAP_MAP_SHMEM 0xbf /* Owned by shmem/tmpfs, in first swap_map */
>
> /*
> * The in-memory structure used to track swap areas.
> @@ -315,6 +316,7 @@ extern swp_entry_t get_swap_page(void);
> extern swp_entry_t get_swap_page_of_type(int);
> extern int valid_swaphandles(swp_entry_t, unsigned long *);
> extern int add_swap_count_continuation(swp_entry_t, gfp_t);
> +extern void swap_shmem_alloc(swp_entry_t);
> extern int swap_duplicate(swp_entry_t);
> extern int swapcache_prepare(swp_entry_t);
> extern void swap_free(swp_entry_t);
> @@ -393,6 +395,10 @@ static inline int add_swap_count_continu
> return 0;
> }
>
> +static inline void swap_shmem_alloc(swp_entry_t swp)
> +{
> +}
> +
> static inline int swap_duplicate(swp_entry_t swp)
> {
> return 0;
> --- si7/mm/shmem.c 2009-09-28 00:28:41.000000000 +0100
> +++ si8/mm/shmem.c 2009-10-14 21:27:07.000000000 +0100
> @@ -1017,7 +1017,14 @@ int shmem_unuse(swp_entry_t entry, struc
> goto out;
> }
> mutex_unlock(&shmem_swaplist_mutex);
> -out: return found; /* 0 or 1 or -ENOMEM */
> + /*
> + * Can some race bring us here? We've been holding page lock,
> + * so I think not; but would rather try again later than BUG()
> + */
> + unlock_page(page);
> + page_cache_release(page);
> +out:
> + return (found < 0) ? found : 0;
> }
>
> /*
> @@ -1080,7 +1087,7 @@ static int shmem_writepage(struct page *
> else
> inode = NULL;
> spin_unlock(&info->lock);
> - swap_duplicate(swap);
> + swap_shmem_alloc(swap);
> BUG_ON(page_mapped(page));
> page_cache_release(page); /* pagecache ref */
> swap_writepage(page, wbc);
> --- si7/mm/swapfile.c 2009-10-14 21:26:57.000000000 +0100
> +++ si8/mm/swapfile.c 2009-10-14 21:27:07.000000000 +0100
> @@ -548,6 +548,12 @@ static unsigned char swap_entry_free(str
> if (usage == SWAP_HAS_CACHE) {
> VM_BUG_ON(!has_cache);
> has_cache = 0;
> + } else if (count == SWAP_MAP_SHMEM) {
> + /*
> + * Or we could insist on shmem.c using a special
> + * swap_shmem_free() and free_shmem_swap_and_cache()...
> + */
> + count = 0;
> } else if ((count & ~COUNT_CONTINUED) <= SWAP_MAP_MAX) {
> if (count == COUNT_CONTINUED) {
> if (swap_count_continued(p, offset, count))
> @@ -1031,7 +1037,6 @@ static int try_to_unuse(unsigned int typ
> swp_entry_t entry;
> unsigned int i = 0;
> int retval = 0;
> - int shmem;
>
> /*
> * When searching mms for an entry, a good strategy is to
> @@ -1107,17 +1112,18 @@ static int try_to_unuse(unsigned int typ
>
> /*
> * Remove all references to entry.
> - * Whenever we reach init_mm, there's no address space
> - * to search, but use it as a reminder to search shmem.
> */
> - shmem = 0;
> swcount = *swap_map;
> - if (swap_count(swcount)) {
> - if (start_mm == &init_mm)
> - shmem = shmem_unuse(entry, page);
> - else
> - retval = unuse_mm(start_mm, entry, page);
> + if (swap_count(swcount) == SWAP_MAP_SHMEM) {
> + retval = shmem_unuse(entry, page);
> + /* page has already been unlocked and released */
> + if (retval < 0)
> + break;
> + continue;
> }
> + if (swap_count(swcount) && start_mm != &init_mm)
> + retval = unuse_mm(start_mm, entry, page);
> +
> if (swap_count(*swap_map)) {
> int set_start_mm = (*swap_map >= swcount);
> struct list_head *p = &start_mm->mmlist;
> @@ -1128,7 +1134,7 @@ static int try_to_unuse(unsigned int typ
> atomic_inc(&new_start_mm->mm_users);
> atomic_inc(&prev_mm->mm_users);
> spin_lock(&mmlist_lock);
> - while (swap_count(*swap_map) && !retval && !shmem &&
> + while (swap_count(*swap_map) && !retval &&
> (p = p->next) != &start_mm->mmlist) {
> mm = list_entry(p, struct mm_struct, mmlist);
> if (!atomic_inc_not_zero(&mm->mm_users))
> @@ -1142,10 +1148,9 @@ static int try_to_unuse(unsigned int typ
> swcount = *swap_map;
> if (!swap_count(swcount)) /* any usage ? */
> ;
> - else if (mm == &init_mm) {
> + else if (mm == &init_mm)
> set_start_mm = 1;
> - shmem = shmem_unuse(entry, page);
> - } else
> + else
> retval = unuse_mm(mm, entry, page);
>
> if (set_start_mm &&
> @@ -1162,13 +1167,6 @@ static int try_to_unuse(unsigned int typ
> mmput(start_mm);
> start_mm = new_start_mm;
> }
> - if (shmem) {
> - /* page has already been unlocked and released */
> - if (shmem > 0)
> - continue;
> - retval = shmem;
> - break;
> - }
> if (retval) {
> unlock_page(page);
> page_cache_release(page);
> @@ -2128,6 +2126,15 @@ bad_file:
> }
>
> /*
> + * Help swapoff by noting that swap entry belongs to shmem/tmpfs
> + * (in which case its reference count is never incremented).
> + */
> +void swap_shmem_alloc(swp_entry_t entry)
> +{
> + __swap_duplicate(entry, SWAP_MAP_SHMEM);
> +}
> +
> +/*
> * increase reference count of swap entry by 1.
> */
> int swap_duplicate(swp_entry_t entry)
>
> --
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>



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