[PATCH v6 19/19] btrfs: try more times to alloc metadata reserve space

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

From: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>

In btrfs_delalloc_reserve_metadata(), the number of metadata bytes we try
to reserve is calculated by the difference between outstanding_extents and
reserved_extents.

When reserve_metadata_bytes() fails to reserve desited metadata space,
it has already done some reclaim work, such as write ordered extents.

In that case, outstanding_extents and reserved_extents may already
changed, and we may reserve enough metadata space then.

So this patch will try to call reserve_metadata_bytes() at most 3 times
to ensure we really run out of space.

Such false ENOSPC is mainly caused by small file extents and time
consuming delalloc functions, which mainly affects in-band
de-duplication. (Compress should also be affected, but LZO/zlib is
faster than SHA256, so still harder to trigger than dedup).

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
---
 fs/btrfs/extent-tree.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 2a17c88..c60e24a 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -5669,6 +5669,7 @@ int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
 	bool delalloc_lock = true;
 	u64 to_free = 0;
 	unsigned dropped;
+	int loops = 0;
 
 	/* If we are a free space inode we need to not flush since we will be in
 	 * the middle of a transaction commit.  We also don't need the delalloc
@@ -5684,11 +5685,12 @@ int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
 	    btrfs_transaction_in_commit(root->fs_info))
 		schedule_timeout(1);
 
+	num_bytes = ALIGN(num_bytes, root->sectorsize);
+
+again:
 	if (delalloc_lock)
 		mutex_lock(&BTRFS_I(inode)->delalloc_mutex);
 
-	num_bytes = ALIGN(num_bytes, root->sectorsize);
-
 	spin_lock(&BTRFS_I(inode)->lock);
 	nr_extents = (unsigned)div64_u64(num_bytes +
 					 BTRFS_MAX_EXTENT_SIZE - 1,
@@ -5809,6 +5811,23 @@ out_fail:
 	}
 	if (delalloc_lock)
 		mutex_unlock(&BTRFS_I(inode)->delalloc_mutex);
+	/*
+	 * The number of metadata bytes is calculated by the difference
+	 * between outstanding_extents and reserved_extents. Sometimes though
+	 * reserve_metadata_bytes() fails to reserve the wanted metadata bytes,
+	 * indeed it has already done some work to reclaim metadata space, hence
+	 * both outstanding_extents and reserved_extents would have changed and
+	 * the bytes we try to reserve would also has changed(may be smaller).
+	 * So here we try to reserve again. This is much useful for online
+	 * dedup, which will easily eat almost all meta space.
+	 *
+	 * XXX: Indeed here 3 is arbitrarily choosed, it's a good workaround for
+	 * online dedup, later we should find a better method to avoid dedup
+	 * enospc issue.
+	 */
+	if (unlikely(ret == -ENOSPC && loops++ < 3))
+		goto again;
+
 	return ret;
 }
 
-- 
2.7.0