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    Subject[PATCH v4 4/9] mm/swap: skip read ahead for unreferenced swap slots
    Date
    We can avoid needlessly allocating page for swap slots that
    are not used by anyone. No pages have to be read in for
    these slots.

    Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
    Co-developed-by: "Huang, Ying" <ying.huang@intel.com>
    ---
    include/linux/swap.h | 6 ++++++
    mm/swap_state.c | 4 ++++
    mm/swapfile.c | 47 +++++++++++++++++++++++++++++++++++++++++------
    3 files changed, 51 insertions(+), 6 deletions(-)

    diff --git a/include/linux/swap.h b/include/linux/swap.h
    index 5f66c84..6b7a48b 100644
    --- a/include/linux/swap.h
    +++ b/include/linux/swap.h
    @@ -424,6 +424,7 @@ extern unsigned int count_swap_pages(int, int);
    extern sector_t map_swap_page(struct page *, struct block_device **);
    extern sector_t swapdev_block(int, pgoff_t);
    extern int page_swapcount(struct page *);
    +extern int __swp_swapcount(swp_entry_t entry);
    extern int swp_swapcount(swp_entry_t entry);
    extern struct swap_info_struct *page_swap_info(struct page *);
    extern bool reuse_swap_page(struct page *, int *);
    @@ -518,6 +519,11 @@ static inline int page_swapcount(struct page *page)
    return 0;
    }

    +static inline int __swp_swapcount(swp_entry_t entry)
    +{
    + return 0;
    +}
    +
    static inline int swp_swapcount(swp_entry_t entry)
    {
    return 0;
    diff --git a/mm/swap_state.c b/mm/swap_state.c
    index 3863acd..3d76d80 100644
    --- a/mm/swap_state.c
    +++ b/mm/swap_state.c
    @@ -323,6 +323,10 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
    if (found_page)
    break;

    + /* Just skip read ahead for unused swap slot */
    + if (!__swp_swapcount(entry))
    + return NULL;
    +
    /*
    * Get a new page to read into from swap.
    */
    diff --git a/mm/swapfile.c b/mm/swapfile.c
    index eb6cba7..136c9dd 100644
    --- a/mm/swapfile.c
    +++ b/mm/swapfile.c
    @@ -803,7 +803,7 @@ swp_entry_t get_swap_page_of_type(int type)
    return (swp_entry_t) {0};
    }

    -static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
    +static struct swap_info_struct *__swap_info_get(swp_entry_t entry)
    {
    struct swap_info_struct *p;
    unsigned long offset, type;
    @@ -819,13 +819,8 @@ static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
    offset = swp_offset(entry);
    if (offset >= p->max)
    goto bad_offset;
    - if (!p->swap_map[offset])
    - goto bad_free;
    return p;

    -bad_free:
    - pr_err("swap_info_get: %s%08lx\n", Unused_offset, entry.val);
    - goto out;
    bad_offset:
    pr_err("swap_info_get: %s%08lx\n", Bad_offset, entry.val);
    goto out;
    @@ -838,6 +833,24 @@ static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
    return NULL;
    }

    +static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
    +{
    + struct swap_info_struct *p;
    +
    + p = __swap_info_get(entry);
    + if (!p)
    + goto out;
    + if (!p->swap_map[swp_offset(entry)])
    + goto bad_free;
    + return p;
    +
    +bad_free:
    + pr_err("swap_info_get: %s%08lx\n", Unused_offset, entry.val);
    + goto out;
    +out:
    + return NULL;
    +}
    +
    static struct swap_info_struct *swap_info_get(swp_entry_t entry)
    {
    struct swap_info_struct *p;
    @@ -993,6 +1006,28 @@ int page_swapcount(struct page *page)

    /*
    * How many references to @entry are currently swapped out?
    + * This does not give an exact answer when swap count is continued,
    + * but does include the high COUNT_CONTINUED flag to allow for that.
    + */
    +int __swp_swapcount(swp_entry_t entry)
    +{
    + int count = 0;
    + pgoff_t offset;
    + struct swap_info_struct *si;
    + struct swap_cluster_info *ci;
    +
    + si = __swap_info_get(entry);
    + if (si) {
    + offset = swp_offset(entry);
    + ci = lock_cluster_or_swap_info(si, offset);
    + count = swap_count(si->swap_map[offset]);
    + unlock_cluster_or_swap_info(si, ci);
    + }
    + return count;
    +}
    +
    +/*
    + * How many references to @entry are currently swapped out?
    * This considers COUNT_CONTINUED so it returns exact answer.
    */
    int swp_swapcount(swp_entry_t entry)
    --
    2.5.5
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