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.

[akpm@linux-foundation.org: fix it for page_cluster >= 32]
Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Adrian Drzewiecki <z@drze.net>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 0 insertions, 52 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 44595a3..b82c028 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2288,58 +2288,6 @@ 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.
- */
-int valid_swaphandles(swp_entry_t entry, unsigned long *offset)
-{
-	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++;
-
-	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;
-	}
-	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;
-}
-
-/*
  * add_swap_count_continuation - called when a swap count is duplicated
  * beyond SWAP_MAP_MAX, it allocates a new page and links that to the entry's
  * page of the original vmalloc'ed swap_map, to hold the continuation count