swapin: fix valid_swaphandles defect

valid_swaphandles is supposed to do a quick pass over the swap map entries
neigbouring the entry which swapin_readahead is targetting, to determine for
it a range worth reading all together.  But since it always starts its search
from the beginning of the swap "cluster", a reject (free entry) there
immediately curtails the readaround, and every swapin_readahead from that
cluster is for just a single page.  Instead scan forwards and backwards around
the target entry.

Use better names for some variables: a swap_info pointer is usually called
"si" not "swapdev".  And at the end, if only the target page should be read,
return count of 0 to disable readaround, to avoid the unnecessarily repeated
call to read_swap_cache_async.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/swapfile.c b/mm/swapfile.c
index ab93505dfbf47ebb228fc17da189758c3367ad68..f5ba723faf819764833e4a2c415f7939214ff79f 100644 (file)
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1769,31 +1769,48 @@ get_swap_info_struct(unsigned type)
  */
 int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
 {
+       struct swap_info_struct *si;
        int our_page_cluster = page_cluster;
-       int ret = 0, i = 1 << our_page_cluster;
-       unsigned long toff;
-       struct swap_info_struct *swapdev = swp_type(entry) + swap_info;
+       pgoff_t target, toff;
+       pgoff_t base, end;
+       int nr_pages = 0;
 
        if (!our_page_cluster)  /* no readahead */
                return 0;
-       toff = (swp_offset(entry) >> our_page_cluster) << our_page_cluster;
-       if (!toff)              /* first page is swap header */
-               toff++, i--;
-       *offset = toff;
+
+       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++;
 
        spin_lock(&swap_lock);
-       do {
-               /* Don't read-ahead past the end of the swap area */
-               if (toff >= swapdev->max)
+       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 (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 (!swapdev->swap_map[toff])
+               if (!si->swap_map[toff])
                        break;
-               if (swapdev->swap_map[toff] == SWAP_MAP_BAD)
+               if (si->swap_map[toff] == SWAP_MAP_BAD)
                        break;
-               toff++;
-               ret++;
-       } while (--i);
+       }
        spin_unlock(&swap_lock);
-       return ret;
+
+       /*
+        * 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;
 }