If right after starting the snapshot creation ioctl we perform a write against a
file followed by a truncate, with both operations increasing the file's size, we
can get a snapshot tree that reflects a state of the source subvolume's tree
where
the file truncation happened but the write operation didn't. This leaves a gap
between 2 file extent items of the inode, which makes btrfs' fsck complain
about it.
For example, if we perform the following file operations:
$ mkfs.btrfs -f /dev/vdd
$ mount /dev/vdd /mnt
$ xfs_io -f \
-c "pwrite -S 0xaa -b 32K 0 32K" \
-c "fsync" \
-c "pwrite -S 0xbb -b 32770 16K 32770" \
-c "truncate 90123" \
/mnt/foobar
and the snapshot creation ioctl was just called before the second write, we
often
can get the following inode items in the snapshot's btree:
item 120 key (257 INODE_ITEM 0) itemoff 7987 itemsize 160
inode generation 146 transid 7 size 90123 block group 0 mode
100600 links 1 uid 0 gid 0 rdev 0 flags 0x0
item 121 key (257 INODE_REF 256) itemoff 7967 itemsize 20
inode ref index 282 namelen 10 name: foobar
item 122 key (257 EXTENT_DATA 0) itemoff 7914 itemsize 53
extent data disk byte 1104855040 nr 32768
extent data offset 0 nr 32768 ram 32768
extent compression 0
item 123 key (257 EXTENT_DATA 53248) itemoff 7861 itemsize 53
extent data disk byte 0 nr 0
extent data offset 0 nr 40960 ram 40960
extent compression 0
There's a file range, corresponding to the interval [32K; ALIGN(16K + 32770,
4096)[
for which there's no file extent item covering it. This is because the file
write
and file truncate operations happened both right after the snapshot creation
ioctl
called btrfs_start_delalloc_inodes(), which means we didn't start and wait for
the
ordered extent that matches the write and, in btrfs_setsize(), we were able to
call
btrfs_cont_expand() before being able to commit the current transaction in the
snapshot creation ioctl. So this made it possibe to insert the hole file extent
item in the source subvolume (which represents the region added by the truncate)
right before the transaction commit from the snapshot creation ioctl.
Btrfs' fsck tool complains about such cases with a message like the following:
"root 331 inode 257 errors 100, file extent discount"
>From a user perspective, the expectation when a snapshot is created while those
file operations are being performed is that the snapshot will have a file that
either:
1) is empty
2) only the first write was captured
3) only the 2 writes were captured
4) both writes and the truncation were captured
But never capture a state where only the first write and the truncation were
captured (since the second write was performed before the truncation).
A test case for xfstests follows.
Signed-off-by: Filipe Manana <fdman...@suse.com>
---
V2: Use different approach to solve the problem. Don't start and wait for all
dellaloc to finish after every expanding truncate, instead add an additional
flush at transaction commit time if we're doing a transaction commit that
creates snapshots.
fs/btrfs/transaction.c | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 59 insertions(+)
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 396ae8b..18c356e 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1714,12 +1714,65 @@ static inline void btrfs_wait_delalloc_flush(struct
btrfs_fs_info *fs_info)
btrfs_wait_ordered_roots(fs_info, -1);
}
+static int
+start_pending_snapshot_roots_delalloc(struct btrfs_trans_handle *trans,
+ struct list_head *splice)
+{
+ struct btrfs_pending_snapshot *pending_snapshot;
+ int ret = 0;
+
+ if (btrfs_test_opt(trans->root, FLUSHONCOMMIT))
+ return 0;
+
+ spin_lock(&trans->root->fs_info->trans_lock);
+ list_splice_init(&trans->transaction->pending_snapshots, splice);
+ spin_unlock(&trans->root->fs_info->trans_lock);
+
+ /*
+ * Start again delalloc for the roots our pending snapshots are made
+ * from. We did it before starting/joining a transaction and we do it
+ * here again because new inode operations might have happened since
+ * then and we want to make sure the snapshot captures a fully
+ * consistent state of the source root tree. For example, if after the
+ * first delalloc flush a write is made against an inode followed by
+ * an expanding truncate, we want to make sure the snapshot captured
+ * both the write and the truncation, and not just the truncation.
+ * Here we shouldn't have much delalloc work to do, as the bulk of it
+ * was done before and outside the transaction.
+ */
+ list_for_each_entry(pending_snapshot, splice, list) {
+ ret = btrfs_start_delalloc_inodes(pending_snapshot->root, 1);
+ if (ret)
+ break;
+ }
+
+ return ret;
+}
+
+static void
+wait_pending_snapshot_roots_delalloc(struct btrfs_trans_handle *trans,
+ struct list_head *splice)
+{
+ struct btrfs_pending_snapshot *pending_snapshot;
+
+ if (list_empty(splice) || btrfs_test_opt(trans->root, FLUSHONCOMMIT))
+ return;
+
+ list_for_each_entry(pending_snapshot, splice, list)
+ btrfs_wait_ordered_extents(pending_snapshot->root, -1);
+
+ spin_lock(&trans->root->fs_info->trans_lock);
+ list_splice_init(splice, &trans->transaction->pending_snapshots);
+ spin_unlock(&trans->root->fs_info->trans_lock);
+}
+
int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
struct btrfs_root *root)
{
struct btrfs_transaction *cur_trans = trans->transaction;
struct btrfs_transaction *prev_trans = NULL;
struct btrfs_inode *btree_ino = BTRFS_I(root->fs_info->btree_inode);
+ LIST_HEAD(pending_snapshots);
int ret;
/* Stop the commit early if ->aborted is set */
@@ -1802,6 +1855,10 @@ int btrfs_commit_transaction(struct btrfs_trans_handle
*trans,
if (ret)
goto cleanup_transaction;
+ ret = start_pending_snapshot_roots_delalloc(trans, &pending_snapshots);
+ if (ret)
+ goto cleanup_transaction;
+
ret = btrfs_run_delayed_items(trans, root);
if (ret)
goto cleanup_transaction;
@@ -1816,6 +1873,8 @@ int btrfs_commit_transaction(struct btrfs_trans_handle
*trans,
btrfs_wait_delalloc_flush(root->fs_info);
+ wait_pending_snapshot_roots_delalloc(trans, &pending_snapshots);
+
btrfs_scrub_pause(root);
/*
* Ok now we need to make sure to block out any other joins while we
--
1.9.1
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