[PATCH 24/27] libxfs: add a buftarg cache shrinker implementation

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

Add a list_lru scanner that runs from the memory pressure detection
to free an amount of the buffer cache that will keep the cache from
growing when there is memory pressure.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 libxfs/buftarg.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/libxfs/buftarg.c b/libxfs/buftarg.c
index 6c7142d41eb1..8332bf3341b6 100644
--- a/libxfs/buftarg.c
+++ b/libxfs/buftarg.c
@@ -62,6 +62,19 @@ xfs_buftarg_setsize_early(
 	return xfs_buftarg_setsize(btp, bsize);
 }
 
+static void
+dispose_list(
+	struct list_head	*dispose)
+{
+	struct xfs_buf		*bp;
+
+	while (!list_empty(dispose)) {
+		bp = list_first_entry(dispose, struct xfs_buf, b_lru);
+		list_del_init(&bp->b_lru);
+		xfs_buf_rele(bp);
+	}
+}
+
 /*
  * Scan a chunk of the buffer cache and drop LRU reference counts. If the
  * count goes to zero, dispose of the buffer.
@@ -70,6 +83,13 @@ static void
 xfs_buftarg_shrink(
 	struct xfs_buftarg	*btc)
 {
+	struct list_lru		*lru = &btc->bt_lru;
+	struct xfs_buf		*bp;
+	int			count;
+	int			progress = 16384;
+	int			rotate = 0;
+	LIST_HEAD(dispose);
+
 	/*
 	 * Make the fact we are in memory reclaim externally visible. This
 	 * allows buffer cache allocation throttling while we are trying to
@@ -79,6 +99,37 @@ xfs_buftarg_shrink(
 
 	fprintf(stderr, "Got memory pressure event. Shrinking caches!\n");
 
+	spin_lock(&lru->l_lock);
+	count = lru->l_count / 50;	/* 2% */
+	fprintf(stderr, "cache size before %ld/%d\n", lru->l_count, count);
+	while (count-- > 0 && !list_empty(&lru->l_lru)) {
+		bp = list_first_entry(&lru->l_lru, struct xfs_buf, b_lru);
+		spin_lock(&bp->b_lock);
+		if (!atomic_add_unless(&bp->b_lru_ref, -1, 1)) {
+			atomic_set(&bp->b_lru_ref, 0);
+			bp->b_state |= XFS_BSTATE_DISPOSE;
+			list_move(&bp->b_lru, &dispose);
+			lru->l_count--;
+		} else {
+			rotate++;
+			list_move_tail(&bp->b_lru, &lru->l_lru);
+		}
+
+		spin_unlock(&bp->b_lock);
+		if (--progress == 0) {
+			fprintf(stderr, "Disposing! rotated %d, lru %ld\n", rotate, lru->l_count);
+			spin_unlock(&lru->l_lock);
+			dispose_list(&dispose);
+			spin_lock(&lru->l_lock);
+			progress = 16384;
+			rotate = 0;
+		}
+	}
+	spin_unlock(&lru->l_lock);
+
+	dispose_list(&dispose);
+	fprintf(stderr, "cache size after %ld, count remaining %d\n", lru->l_count, count);
+
 	/*
 	 * Now we've free a bunch of memory, trim the heap down to release the
 	 * freed memory back to the kernel and reduce the pressure we are
-- 
2.28.0