Btrfs: fix enospc problems with delalloc

So I had this brilliant idea to use atomic counters for outstanding and reserved
extents, but this turned out to be a bad idea.  Consider this where we have 1
outstanding extent and 1 reserved extent

Reserver				Releaser
					atomic_dec(outstanding) now 0
atomic_read(outstanding)+1 get 1
atomic_read(reserved) get 1
don't actually reserve anything because
they are the same
					atomic_cmpxchg(reserved, 1, 0)
atomic_inc(outstanding)
atomic_add(0, reserved)
					free reserved space for 1 extent

Then the reserver now has no actual space reserved for it, and when it goes to
finish the ordered IO it won't have enough space to do it's allocation and you
get those lovely warnings.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 49 insertions, 41 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 0a5bd67e2894..d7031e7dfd76 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3726,7 +3726,6 @@ int btrfs_block_rsv_check(struct btrfs_trans_handle *trans,
         if (commit_trans) {
                 if (trans)
                         return -EAGAIN;
-
                 trans = btrfs_join_transaction(root);
                 BUG_ON(IS_ERR(trans));
                 ret = btrfs_commit_transaction(trans, root);
@@ -3946,6 +3945,30 @@ int btrfs_snap_reserve_metadata(struct btrfs_trans_handle *trans,
         return block_rsv_migrate_bytes(src_rsv, dst_rsv, num_bytes);
 }
 
+static unsigned drop_outstanding_extent(struct inode *inode)
+{
+        unsigned dropped_extents = 0;
+
+        spin_lock(&BTRFS_I(inode)->lock);
+        BUG_ON(!BTRFS_I(inode)->outstanding_extents);
+        BTRFS_I(inode)->outstanding_extents--;
+
+        /*
+         * If we have more or the same amount of outsanding extents than we have
+         * reserved then we need to leave the reserved extents count alone.
+         */
+        if (BTRFS_I(inode)->outstanding_extents >=
+            BTRFS_I(inode)->reserved_extents)
+                goto out;
+
+        dropped_extents = BTRFS_I(inode)->reserved_extents -
+                BTRFS_I(inode)->outstanding_extents;
+        BTRFS_I(inode)->reserved_extents -= dropped_extents;
+out:
+        spin_unlock(&BTRFS_I(inode)->lock);
+        return dropped_extents;
+}
+
 static u64 calc_csum_metadata_size(struct inode *inode, u64 num_bytes)
 {
         return num_bytes >>= 3;
@@ -3955,9 +3978,8 @@ int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
 {
         struct btrfs_root *root = BTRFS_I(inode)->root;
         struct btrfs_block_rsv *block_rsv = &root->fs_info->delalloc_block_rsv;
-        u64 to_reserve;
-        int nr_extents;
-        int reserved_extents;
+        u64 to_reserve = 0;
+        unsigned nr_extents = 0;
         int ret;
 
         if (btrfs_transaction_in_commit(root->fs_info))
@@ -3965,24 +3987,31 @@ int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
 
         num_bytes = ALIGN(num_bytes, root->sectorsize);
 
-        nr_extents = atomic_read(&BTRFS_I(inode)->outstanding_extents) + 1;
-        reserved_extents = atomic_read(&BTRFS_I(inode)->reserved_extents);
+        spin_lock(&BTRFS_I(inode)->lock);
+        BTRFS_I(inode)->outstanding_extents++;
+
+        if (BTRFS_I(inode)->outstanding_extents >
+            BTRFS_I(inode)->reserved_extents) {
+                nr_extents = BTRFS_I(inode)->outstanding_extents -
+                        BTRFS_I(inode)->reserved_extents;
+                BTRFS_I(inode)->reserved_extents += nr_extents;
 
-        if (nr_extents > reserved_extents) {
-                nr_extents -= reserved_extents;
                 to_reserve = btrfs_calc_trans_metadata_size(root, nr_extents);
-        } else {
-                nr_extents = 0;
-                to_reserve = 0;
         }
+        spin_unlock(&BTRFS_I(inode)->lock);
 
         to_reserve += calc_csum_metadata_size(inode, num_bytes);
         ret = reserve_metadata_bytes(NULL, root, block_rsv, to_reserve, 1);
-        if (ret)
+        if (ret) {
+                unsigned dropped;
+                /*
+                 * We don't need the return value since our reservation failed,
+                 * we just need to clean up our counter.
+                 */
+                dropped = drop_outstanding_extent(inode);
+                WARN_ON(dropped > 1);
                 return ret;
-
-        atomic_add(nr_extents, &BTRFS_I(inode)->reserved_extents);
-        atomic_inc(&BTRFS_I(inode)->outstanding_extents);
+        }
 
         block_rsv_add_bytes(block_rsv, to_reserve, 1);
 
@@ -3992,36 +4021,15 @@ int btrfs_delalloc_reserve_metadata(struct inode *inode, u64 num_bytes)
 void btrfs_delalloc_release_metadata(struct inode *inode, u64 num_bytes)
 {
         struct btrfs_root *root = BTRFS_I(inode)->root;
-        u64 to_free;
-        int nr_extents;
-        int reserved_extents;
+        u64 to_free = 0;
+        unsigned dropped;
 
         num_bytes = ALIGN(num_bytes, root->sectorsize);
-        atomic_dec(&BTRFS_I(inode)->outstanding_extents);
-        WARN_ON(atomic_read(&BTRFS_I(inode)->outstanding_extents) < 0);
-
-        reserved_extents = atomic_read(&BTRFS_I(inode)->reserved_extents);
-        do {
-                int old, new;
-
-                nr_extents = atomic_read(&BTRFS_I(inode)->outstanding_extents);
-                if (nr_extents >= reserved_extents) {
-                        nr_extents = 0;
-                        break;
-                }
-                old = reserved_extents;
-                nr_extents = reserved_extents - nr_extents;
-                new = reserved_extents - nr_extents;
-                old = atomic_cmpxchg(&BTRFS_I(inode)->reserved_extents,
-                                     reserved_extents, new);
-                if (likely(old == reserved_extents))
-                        break;
-                reserved_extents = old;
-        } while (1);
+        dropped = drop_outstanding_extent(inode);
 
         to_free = calc_csum_metadata_size(inode, num_bytes);
-        if (nr_extents > 0)
-                to_free += btrfs_calc_trans_metadata_size(root, nr_extents);
+        if (dropped > 0)
+                to_free += btrfs_calc_trans_metadata_size(root, dropped);
 
         btrfs_block_rsv_release(root, &root->fs_info->delalloc_block_rsv,
                                 to_free);