From 879de98ead5106ffd5486aa6c11a3fad141049d9 Mon Sep 17 00:00:00 2001
From: Dave Chinner <dchinner@redhat.com>
Date: Sun, 13 May 2018 23:10:05 -0700
Subject: [PATCH] xfs: one-shot cached buffers
Git-commit: 879de98ead5106ffd5486aa6c11a3fad141049d9
Patch-mainline: v4.18-rc1
References: bsc#1191500

For the new growfs work, we want to ensure that we serialise
secondary superblock updates with other operations (e.g. scrub)
correctly, but we don't want to cache the buffers for long term
reuse. We need cached buffers for serialisation, however.

To solve this, introduce a "oneshot" buffer which will be marshalled
through the cache but then released once the last current reference
goes away. If the buffer is already cached, then we ignore the
"one-shot" behaviour and leave the buffer in the state it was prior
to the one-shot command being run. This means we don't perturb
either the working set or existing cached buffer state by a one-shot
operation.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: Anthony Iliopoulos <ailiop@suse.com>

---
 fs/xfs/xfs_buf.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
index 830e2f6c064a..f5f2b71c2fde 100644
--- a/fs/xfs/xfs_buf.h
+++ b/fs/xfs/xfs_buf.h
@@ -347,6 +347,18 @@ extern void xfs_buf_terminate(void);
 
 void xfs_buf_set_ref(struct xfs_buf *bp, int lru_ref);
 
+/*
+ * If the buffer is already on the LRU, do nothing. Otherwise set the buffer
+ * up with a reference count of 0 so it will be tossed from the cache when
+ * released.
+ */
+static inline void xfs_buf_oneshot(struct xfs_buf *bp)
+{
+	if (!list_empty(&bp->b_lru) || atomic_read(&bp->b_lru_ref) > 1)
+		return;
+	atomic_set(&bp->b_lru_ref, 0);
+}
+
 static inline int xfs_buf_ispinned(struct xfs_buf *bp)
 {
 	return atomic_read(&bp->b_pin_count);
-- 
2.33.0