[PATCH V2] Btrfs: really fix trim 0 bytes after a device delete

[lutz.euler@freenet.de (Lutz Euler)]

Commit 2cac13e41bf5b99ffc426bd28dfd2248df1dfa67, "fix trim 0 bytes after
a device delete", said:
  A user reported a bug of btrfs's trim, that is we will trim 0 bytes
  after a device delete.
The commit didn't attack the root of the problem so did not fix the bug
except for a special case.

For block discard, btrfs_trim_fs directly compares the range passed in
against the filesystem's objectids. The former is bounded by the sum of
the sizes of the devices of the filesystem, the latter is a completely
unrelated set of intervals of 64-bit integers. The bug reported occurred
as the smallest objectid was larger than the sum of the device sizes.
The above mentioned commit only fixed the case where the smallest
objectid is nonzero and the largest objectid less than the sum of the
device sizes, but it still trims too little if the largest objectid is
larger than that, and nothing in the reported situation.

The current mapping between the given range and the objectids is thus
clearly broken, so, to fix the bug and as a first step towards a
complete solution, simply ignore the range parameter's start and length
fields and always trim the whole filesystem. (While this makes it
impossible to trim a filesystem only partly, due to the broken mapping
this often didn't work anyway.)

V2:
- Rebased onto 3.9. (still applies to and works with 3.19-rc2)
- Take range->minlen into account.

Reported-by: Lutz Euler <lutz.euler@freenet.de>
Signed-off-by: Lutz Euler <lutz.euler@freenet.de>
---
 fs/btrfs/extent-tree.c |   25 +++++++++++--------------
 1 files changed, 11 insertions(+), 14 deletions(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index cfb3cf7..81006c1 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -8824,26 +8824,23 @@ int btrfs_trim_fs(struct btrfs_root *root, struct fstrim_range *range)
 	u64 start;
 	u64 end;
 	u64 trimmed = 0;
-	u64 total_bytes = btrfs_super_total_bytes(fs_info->super_copy);
 	int ret = 0;
 
 	/*
-	 * try to trim all FS space, our block group may start from non-zero.
+	 * The range passed in is a subinterval of the interval from 0
+	 * to the sum of the sizes of the devices of the filesystem.
+	 * The objectid's used in the filesystem can span any set of
+	 * subintervals of the interval from 0 to (u64)-1. As there is
+	 * neither a simple nor an agreed upon mapping between these
+	 * two ranges we ignore the range parameter's start and len
+	 * fields and always trim the whole filesystem (that is, only
+	 * the free space in allocated chunks).
 	 */
-	if (range->len == total_bytes)
-		cache = btrfs_lookup_first_block_group(fs_info, range->start);
-	else
-		cache = btrfs_lookup_block_group(fs_info, range->start);
+	cache = btrfs_lookup_first_block_group(fs_info, 0);
 
 	while (cache) {
-		if (cache->key.objectid >= (range->start + range->len)) {
-			btrfs_put_block_group(cache);
-			break;
-		}
-
-		start = max(range->start, cache->key.objectid);
-		end = min(range->start + range->len,
-				cache->key.objectid + cache->key.offset);
+		start = cache->key.objectid;
+		end = cache->key.objectid + cache->key.offset;
 
 		if (end - start >= range->minlen) {
 			if (!block_group_cache_done(cache)) {
-- 
1.7.4.1