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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)
    >
    > --
    > To unsubscribe, send a message with 'unsubscribe linux-mm' in
    > the body to majordomo@kvack.org. For more info on Linux MM,
    > see: http://www.linux-mm.org/ .
    > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
    >



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