[PATCH v2 15/21] ocfs2: check for a stale write error before reusing a metadata buffer

[Chao Shi <coshi036@gmail.com>]

__ocfs2_journal_access() refuses to journal a buffer whose previous write
failed, and turns the filesystem read-only rather than risk metadata
inconsistency.  That check sits inside an if (!buffer_uptodate(bh)) block,
because until now a failed write also cleared BH_Uptodate.

This series stops clearing BH_Uptodate on write error, so that outer test
would never fire again and ocfs2 would silently start reusing buffers whose
last write failed.  Hoist the check out of the debug block, where it does
not depend on BH_Uptodate any more, and drop the now dead second half of
its condition.

The mlog() pair keeps its own !buffer_uptodate() guard: it is a separate
"we can safely remove this assertion after testing" debug aid about being
handed a buffer with no valid contents, which is a different question from
whether the last write of that buffer failed.

The unlocked test followed by a locked retest is deliberate.  BH_Write_EIO
is cleared under the buffer lock, so taking the lock and looking a second
time avoids turning the
filesystem read-only over an error that a concurrent rewrite has already
cleared, while keeping the common case lock-free.

The code in this patch is Jan's, from the review discussion linked in the
cover letter.

Suggested-by: Jan Kara <jack@suse.cz>
Acked-by: Weidong Zhu <weizhu@fiu.edu>
Signed-off-by: Chao Shi <coshi036@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
---
 fs/ocfs2/journal.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
index d8afbc1a76bb..ea6802d894c2 100644
--- a/fs/ocfs2/journal.c
+++ b/fs/ocfs2/journal.c
@@ -676,19 +676,20 @@ static int __ocfs2_journal_access(handle_t *handle,
 		mlog(ML_ERROR, "giving me a buffer that's not uptodate!\n");
 		mlog(ML_ERROR, "b_blocknr=%llu, b_state=0x%lx\n",
 		     (unsigned long long)bh->b_blocknr, bh->b_state);
-
+	}
+	/*
+	 * A previous transaction with a couple of buffer heads fail
+	 * to checkpoint, so all the bhs are marked as BH_Write_EIO.
+	 * For current transaction, the bh is just among those error
+	 * bhs which previous transaction handle. We can't just clear
+	 * its BH_Write_EIO and reuse directly, since other bhs are
+	 * not written to disk yet and that will cause metadata
+	 * inconsistency. So we should set fs read-only to avoid
+	 * further damage.
+	 */
+	if (buffer_write_io_error(bh)) {
 		lock_buffer(bh);
-		/*
-		 * A previous transaction with a couple of buffer heads fail
-		 * to checkpoint, so all the bhs are marked as BH_Write_EIO.
-		 * For current transaction, the bh is just among those error
-		 * bhs which previous transaction handle. We can't just clear
-		 * its BH_Write_EIO and reuse directly, since other bhs are
-		 * not written to disk yet and that will cause metadata
-		 * inconsistency. So we should set fs read-only to avoid
-		 * further damage.
-		 */
-		if (buffer_write_io_error(bh) && !buffer_uptodate(bh)) {
+		if (buffer_write_io_error(bh)) {
 			unlock_buffer(bh);
 			return ocfs2_error(osb->sb, "A previous attempt to "
 					"write this buffer head failed\n");
-- 
2.43.0