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    Subject[PATCH v2 -mm] make swapin readahead skip over holes
    Ever since abandoning the virtual scan of processes, for scalability
    reasons, swap space has been a little more fragmented than before.
    This can lead to the situation where a large memory user is killed,
    swap space ends up full of "holes" and swapin readahead is totally
    ineffective.

    On my home system, after killing a leaky firefox it took over an
    hour to page just under 2GB of memory back in, slowing the virtual
    machines down to a crawl.

    This patch makes swapin readahead simply skip over holes, instead
    of stopping at them. This allows the system to swap things back in
    at rates of several MB/second, instead of a few hundred kB/second.

    The checks done in valid_swaphandles are already done in
    read_swap_cache_async as well, allowing us to remove a fair amount
    of code.

    Signed-off-by: Rik van Riel <riel@redhat.com>
    ---
    -v2: cleanups suggested by Mel Gorman
    skip swap offset zero

    include/linux/swap.h | 2 +-
    mm/swap_state.c | 17 ++++----------
    mm/swapfile.c | 59 +++++++++++++------------------------------------
    3 files changed, 22 insertions(+), 56 deletions(-)

    diff --git a/include/linux/swap.h b/include/linux/swap.h
    index 1e22e12..6e1282e 100644
    --- a/include/linux/swap.h
    +++ b/include/linux/swap.h
    @@ -328,7 +328,7 @@ extern long total_swap_pages;
    extern void si_swapinfo(struct sysinfo *);
    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 void get_swap_cluster(swp_entry_t, unsigned long *, 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);
    diff --git a/mm/swap_state.c b/mm/swap_state.c
    index ea6b32d..3ec4886 100644
    --- a/mm/swap_state.c
    +++ b/mm/swap_state.c
    @@ -372,25 +372,18 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
    struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
    struct vm_area_struct *vma, unsigned long addr)
    {
    - int nr_pages;
    struct page *page;
    - unsigned long offset;
    + unsigned long offset = swp_offset(entry);
    unsigned long end_offset;

    - /*
    - * Get starting offset for readaround, and number of pages to read.
    - * Adjust starting address by readbehind (for NUMA interleave case)?
    - * No, it's very unlikely that swap layout would follow vma layout,
    - * more likely that neighbouring swap pages came from the same node:
    - * so use the same "addr" to choose the same node for each swap read.
    - */
    - nr_pages = valid_swaphandles(entry, &offset);
    - for (end_offset = offset + nr_pages; offset < end_offset; offset++) {
    + get_swap_cluster(entry, &offset, &end_offset);
    +
    + for (; offset <= end_offset ; offset++) {
    /* Ok, do the async read-ahead now */
    page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
    gfp_mask, vma, addr);
    if (!page)
    - break;
    + continue;
    page_cache_release(page);
    }
    lru_add_drain(); /* Push any new pages onto the LRU now */
    diff --git a/mm/swapfile.c b/mm/swapfile.c
    index b1cd120..f09182e 100644
    --- a/mm/swapfile.c
    +++ b/mm/swapfile.c
    @@ -2289,55 +2289,28 @@ int swapcache_prepare(swp_entry_t entry)
    }

    /*
    - * swap_lock prevents swap_map being freed. Don't grab an extra
    - * reference on the swaphandle, it doesn't matter if it becomes unused.
    + * Return a swap cluster sized and aligned block around offset.
    */
    -int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
    +void get_swap_cluster(swp_entry_t entry, unsigned long *begin,
    + unsigned long *end)
    {
    struct swap_info_struct *si;
    - int our_page_cluster = page_cluster;
    - pgoff_t target, toff;
    - pgoff_t base, end;
    - int nr_pages = 0;
    -
    - if (!our_page_cluster) /* no readahead */
    - return 0;
    -
    - si = swap_info[swp_type(entry)];
    - target = swp_offset(entry);
    - base = (target >> our_page_cluster) << our_page_cluster;
    - end = base + (1 << our_page_cluster);
    - if (!base) /* first page is swap header */
    - base++;
    + unsigned long offset = swp_offset(entry);
    + unsigned long mask = (1 << page_cluster) - 1;

    spin_lock(&swap_lock);
    - if (end > si->max) /* don't go beyond end of map */
    - end = si->max;
    -
    - /* Count contiguous allocated slots above our target */
    - for (toff = target; ++toff < end; nr_pages++) {
    - /* Don't read in free or bad pages */
    - if (!si->swap_map[toff])
    - break;
    - if (swap_count(si->swap_map[toff]) == SWAP_MAP_BAD)
    - break;
    - }
    - /* Count contiguous allocated slots below our target */
    - for (toff = target; --toff >= base; nr_pages++) {
    - /* Don't read in free or bad pages */
    - if (!si->swap_map[toff])
    - break;
    - if (swap_count(si->swap_map[toff]) == SWAP_MAP_BAD)
    - break;
    - }
    + si = swap_info[swp_type(entry)];
    + /* Round the begin down to a page_cluster boundary. */
    + offset &= ~mask;
    + if (!offset) /* First page is swap header. */
    + offset++;
    + *begin = offset;
    + /* Round the end up, but not beyond the end of the swap device. */
    + offset |= mask;
    + if (offset > si->max)
    + offset = si->max;
    + *end = offset;
    spin_unlock(&swap_lock);
    -
    - /*
    - * Indicate starting offset, and return number of pages to get:
    - * if only 1, say 0, since there's then no readahead to be done.
    - */
    - *offset = ++toff;
    - return nr_pages? ++nr_pages: 0;
    }

    /*


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