jbd2: cleanup needed free block estimates when starting a transaction

__jbd2_log_space_left() and jbd_space_needed() were kind of odd.
jbd_space_needed() accounted also credits needed for currently
committing transaction while it didn't account for credits needed for
control blocks.  __jbd2_log_space_left() then accounted for control
blocks as a fraction of free space.  Since results of these two
functions are always only compared against each other, this works
correct but is somewhat strange.  Move the estimates so that
jbd_space_needed() returns number of blocks needed for a transaction
including control blocks and __jbd2_log_space_left() returns free
space in the journal (with the committing transaction already
subtracted).  Rename functions to jbd2_log_space_left() and
jbd2_space_needed() while we are changing them.

Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 26 insertions, 6 deletions

diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index e33e84b3d5c8..7a1f6cd864c8 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1220,7 +1220,6 @@ extern void	jbd2_clear_buffer_revoked_flags(journal_t *journal);
  * transitions on demand.
  */
 
-int __jbd2_log_space_left(journal_t *); /* Called with journal locked */
 int jbd2_log_start_commit(journal_t *journal, tid_t tid);
 int __jbd2_log_start_commit(journal_t *journal, tid_t tid);
 int jbd2_journal_start_commit(journal_t *journal, tid_t *tid);
@@ -1291,16 +1290,37 @@ extern int jbd2_journal_blocks_per_page(struct inode *inode);
 extern size_t journal_tag_bytes(journal_t *journal);
 
 /*
+ * We reserve t_outstanding_credits >> JBD2_CONTROL_BLOCKS_SHIFT for
+ * transaction control blocks.
+ */
+#define JBD2_CONTROL_BLOCKS_SHIFT 5
+
+/*
  * Return the minimum number of blocks which must be free in the journal
  * before a new transaction may be started.  Must be called under j_state_lock.
  */
-static inline int jbd_space_needed(journal_t *journal)
+static inline int jbd2_space_needed(journal_t *journal)
 {
         int nblocks = journal->j_max_transaction_buffers;
-        if (journal->j_committing_transaction)
-                nblocks += atomic_read(&journal->j_committing_transaction->
-                                       t_outstanding_credits);
-        return nblocks;
+        return nblocks + (nblocks >> JBD2_CONTROL_BLOCKS_SHIFT);
+}
+
+/*
+ * Return number of free blocks in the log. Must be called under j_state_lock.
+ */
+static inline unsigned long jbd2_log_space_left(journal_t *journal)
+{
+        /* Allow for rounding errors */
+        unsigned long free = journal->j_free - 32;
+
+        if (journal->j_committing_transaction) {
+                unsigned long committing = atomic_read(&journal->
+                        j_committing_transaction->t_outstanding_credits);
+
+                /* Transaction + control blocks */
+                free -= committing + (committing >> JBD2_CONTROL_BLOCKS_SHIFT);
+        }
+        return free;
 }
 
 /*