[PATCH 4/7] xfs: force inode garbage collection before fallocate when space is low

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

Generally speaking, when a user calls fallocate, they're looking to
preallocate space in a file in the largest contiguous chunks possible.
If free space is low, it's possible that the free space will look
unnecessarily fragmented because there are unlinked inodes that are
holding on to space that we could allocate.  When this happens,
fallocate makes suboptimal allocation decisions for the sake of deleted
files, which doesn't make much sense, so scan the filesystem for dead
items to delete to try to avoid this.

Note that there are a handful of fstests that fill a filesystem, delete
just enough files to allow a single large allocation, and check that
fallocate actually gets the allocation.  These tests regress because the
test runs fallocate before the inode gc has a chance to run, so add this
behavior to maintain as much of the old behavior as possible.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 fs/xfs/xfs_bmap_util.c |   44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)


diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
index d4ceba5370c7..f3fb64cc8a7c 100644
--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -28,6 +28,7 @@
 #include "xfs_icache.h"
 #include "xfs_iomap.h"
 #include "xfs_reflink.h"
+#include "xfs_sb.h"
 
 /* Kernel only BMAP related definitions and functions */
 
@@ -720,6 +721,44 @@ xfs_free_eofblocks(
 	return error;
 }
 
+/*
+ * If we suspect that the target device is full enough that it isn't to be able
+ * to satisfy the entire request, try a non-sync inode inactivation scan to
+ * free up space.  While it's perfectly fine to fill a preallocation request
+ * with a bunch of short extents, we'd prefer to do the inactivation work now
+ * to combat long term fragmentation in new file data.  This is purely for
+ * optimization, so we don't take any blocking locks and we only look for space
+ * that is already on the reclaim list (i.e. we don't zap speculative
+ * preallocations).
+ */
+static int
+xfs_alloc_reclaim_inactive_space(
+	struct xfs_mount	*mp,
+	bool			is_rt,
+	xfs_filblks_t		allocatesize_fsb)
+{
+	struct xfs_perag	*pag;
+	struct xfs_sb		*sbp = &mp->m_sb;
+	xfs_extlen_t		free;
+	xfs_agnumber_t		agno;
+
+	if (is_rt) {
+		if (sbp->sb_frextents * sbp->sb_rextsize >= allocatesize_fsb)
+			return 0;
+	} else {
+		for (agno = 0; agno < mp->m_sb.sb_agcount; agno++) {
+			pag = xfs_perag_get(mp, agno);
+			free = pag->pagf_freeblks;
+			xfs_perag_put(pag);
+
+			if (free >= allocatesize_fsb)
+				return 0;
+		}
+	}
+
+	return xfs_inodegc_free_space(mp, NULL);
+}
+
 int
 xfs_alloc_file_space(
 	struct xfs_inode	*ip,
@@ -804,6 +843,11 @@ xfs_alloc_file_space(
 			rblocks = 0;
 		}
 
+		error = xfs_alloc_reclaim_inactive_space(mp, rt,
+				allocatesize_fsb);
+		if (error)
+			break;
+
 		/*
 		 * Allocate and setup the transaction.
 		 */