[PATCH] btrfs: do not start and wait for delalloc on snapshot roots on transaction commit

* [PATCH] btrfs: do not start and wait for delalloc on snapshot roots on transaction commit
@ 2020-10-27 12:40 fdmanana
  2020-10-27 17:34 ` Josef Bacik
  2020-10-29 12:24 ` David Sterba
  0 siblings, 2 replies; 3+ messages in thread
From: fdmanana @ 2020-10-27 12:40 UTC (permalink / raw)
  To: linux-btrfs

From: Filipe Manana <fdmanana@suse.com>

We do not need anymore to start writeback for delalloc of roots that are
being snapshoted and wait for it to complete. This was done in commit
609e804d771f59 ("Btrfs: fix file corruption after snapshotting due to mix
of buffered/DIO writes") to fix a type of file corruption where files in a
snapshot end up having their i_size updated in a non-ordered way, leaving
implicit file holes, when buffered IO writes that increase a file's size
are followed by direct IO writes that also increase the file's size.

This is not needed anymore because we now have a more generic mechanism
to prevent a non-ordered i_size update since commit 9ddc959e802bf7
("btrfs: use the file extent tree infrastructure"), which addresses this
scenario involving snapshots as well.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/transaction.c | 49 ++++++------------------------------------
 1 file changed, 6 insertions(+), 43 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 52ada47aff50..8f70d7135497 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1956,10 +1956,8 @@ static void btrfs_cleanup_pending_block_groups(struct btrfs_trans_handle *trans)
        }
 }
 
-static inline int btrfs_start_delalloc_flush(struct btrfs_trans_handle *trans)
+static inline int btrfs_start_delalloc_flush(struct btrfs_fs_info *fs_info)
 {
-	struct btrfs_fs_info *fs_info = trans->fs_info;
-
 	/*
 	 * We use writeback_inodes_sb here because if we used
 	 * btrfs_start_delalloc_roots we would deadlock with fs freeze.
@@ -1969,50 +1967,15 @@ static inline int btrfs_start_delalloc_flush(struct btrfs_trans_handle *trans)
 	 * from already being in a transaction and our join_transaction doesn't
 	 * have to re-take the fs freeze lock.
 	 */
-	if (btrfs_test_opt(fs_info, FLUSHONCOMMIT)) {
+	if (btrfs_test_opt(fs_info, FLUSHONCOMMIT))
 		writeback_inodes_sb(fs_info->sb, WB_REASON_SYNC);
-	} else {
-		struct btrfs_pending_snapshot *pending;
-		struct list_head *head = &trans->transaction->pending_snapshots;
-
-		/*
-		 * Flush dellaloc for any root that is going to be snapshotted.
-		 * This is done to avoid a corrupted version of files, in the
-		 * snapshots, that had both buffered and direct IO writes (even
-		 * if they were done sequentially) due to an unordered update of
-		 * the inode's size on disk.
-		 */
-		list_for_each_entry(pending, head, list) {
-			int ret;
-
-			ret = btrfs_start_delalloc_snapshot(pending->root);
-			if (ret)
-				return ret;
-		}
-	}
 	return 0;
 }
 
-static inline void btrfs_wait_delalloc_flush(struct btrfs_trans_handle *trans)
+static inline void btrfs_wait_delalloc_flush(struct btrfs_fs_info *fs_info)
 {
-	struct btrfs_fs_info *fs_info = trans->fs_info;
-
-	if (btrfs_test_opt(fs_info, FLUSHONCOMMIT)) {
+	if (btrfs_test_opt(fs_info, FLUSHONCOMMIT))
 		btrfs_wait_ordered_roots(fs_info, U64_MAX, 0, (u64)-1);
-	} else {
-		struct btrfs_pending_snapshot *pending;
-		struct list_head *head = &trans->transaction->pending_snapshots;
-
-		/*
-		 * Wait for any dellaloc that we started previously for the roots
-		 * that are going to be snapshotted. This is to avoid a corrupted
-		 * version of files in the snapshots that had both buffered and
-		 * direct IO writes (even if they were done sequentially).
-		 */
-		list_for_each_entry(pending, head, list)
-			btrfs_wait_ordered_extents(pending->root,
-						   U64_MAX, 0, U64_MAX);
-	}
 }
 
 int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
@@ -2150,7 +2113,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 
 	extwriter_counter_dec(cur_trans, trans->type);
 
-	ret = btrfs_start_delalloc_flush(trans);
+	ret = btrfs_start_delalloc_flush(fs_info);
 	if (ret)
 		goto cleanup_transaction;
 
@@ -2166,7 +2129,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 	if (ret)
 		goto cleanup_transaction;
 
-	btrfs_wait_delalloc_flush(trans);
+	btrfs_wait_delalloc_flush(fs_info);
 
 	/*
 	 * Wait for all ordered extents started by a fast fsync that joined this
-- 
2.28.0


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