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Subject[Patch 1/2] 2.4.20-pre4/ext3: Handle dirty buffers encountered unexpectedly.
Ext3's internal debugging has always assumed that it was illegal for there to
be parallel IO on a buffer-head which it is trying to modify. That's
reasonable --- if there is an IO collision, we end up with IOs hitting disk
out-of-order wrt the journal, so we lose recovery guarantees.

However, there are two cases where the test is a little over-zealous. If
user space is performing inherently non-transactional writes (eg. tune2fs
adding a label to a live filesystem and writing to the buffered device
superblock location) then we can hit the ext3 assertion.

More seriously, since 2.4.11 the page cache can lock a buffer_head for read
even if the bh is already under journal control. The tune2fs bug is very
rare: there have been no reports of it in Bugzilla or ext3-users lists, and
only one on 2.5 on linux-kernel. But now, a dump(8) on a live filesystem
can also give rise to the same condition, and in testing, dump + fs activity
reproduces the assertion-failure VERY rapidly.

This patch changes the jbd get-write-access code to take the buffer_head
lock before testing the uptodate and dirty state of a bh, and relaxes the
handling of unexpectedly-dirty buffers to be a printk warning, not a
fatal error. The lock will cure the dump(8) interaction, and the warning
means that we will still spot out-of-order writes, while not taking the
whole kernel down if we collide with a tune2fs(8).

The patch also removes a small potential hole in the recovery guarantees. It
is not safe for a transaction to steal buffers from checkpoint mode until after
that transaction has committed. Otherwise, a reboot at the wrong moment might
find the old copy of the buffer in the journal had been removed from the
recovery set before the new copy was written.

--- linux-ext3-2.4merge-2/include/linux/jbd.h.=K0000=.orig Thu Aug 22 22:54:34 2002
+++ linux-ext3-2.4merge-2/include/linux/jbd.h Thu Aug 22 23:43:16 2002
@@ -32,6 +32,14 @@

#define journal_oom_retry 1

+/*
+ * Define JBD_PARANOID_WRITES to cause a kernel BUG() check if ext3
+ * finds a buffer unexpectedly dirty. This is useful for debugging, but
+ * can cause spurious kernel panics if there are applications such as
+ * tune2fs modifying our buffer_heads behind our backs.
+ */
+#undef JBD_PARANOID_WRITES
+
#ifdef CONFIG_JBD_DEBUG
/*
* Define JBD_EXPENSIVE_CHECKING to enable more expensive internal
@@ -730,6 +738,10 @@
schedule(); \
} while (1)

+extern void __jbd_unexpected_dirty_buffer(char *, int, struct journal_head *);
+#define jbd_unexpected_dirty_buffer(jh) \
+ __jbd_unexpected_dirty_buffer(__FUNCTION__, __LINE__, (jh))
+
/*
* is_journal_abort
*
--- linux-ext3-2.4merge-2/fs/jbd/transaction.c.=K0000=.orig Thu Aug 22 22:53:28 2002
+++ linux-ext3-2.4merge-2/fs/jbd/transaction.c Thu Aug 22 23:43:16 2002
@@ -539,76 +539,67 @@
static int
do_get_write_access(handle_t *handle, struct journal_head *jh, int force_copy)
{
+ struct buffer_head *bh;
transaction_t *transaction = handle->h_transaction;
journal_t *journal = transaction->t_journal;
int error;
char *frozen_buffer = NULL;
int need_copy = 0;
-
+ int locked;
+
jbd_debug(5, "buffer_head %p, force_copy %d\n", jh, force_copy);

JBUFFER_TRACE(jh, "entry");
repeat:
+ bh = jh2bh(jh);
+
/* @@@ Need to check for errors here at some point. */

/*
- * AKPM: neither bdflush nor kupdate run with the BKL. There's
- * nothing we can do to prevent them from starting writeout of a
- * BUF_DIRTY buffer at any time. And checkpointing buffers are on
- * BUF_DIRTY. So. We no longer assert that the buffer is unlocked.
- *
- * However. It is very wrong for us to allow ext3 to start directly
- * altering the ->b_data of buffers which may at that very time be
- * undergoing writeout to the client filesystem. This can leave
- * the filesystem in an inconsistent, transient state if we crash.
- * So what we do is to steal the buffer if it is in checkpoint
- * mode and dirty. The journal lock will keep out checkpoint-mode
- * state transitions within journal_remove_checkpoint() and the buffer
- * is locked to keep bdflush/kupdate/whoever away from it as well.
- *
* AKPM: we have replaced all the lock_journal_bh_wait() stuff with a
* simple lock_journal(). This code here will care for locked buffers.
*/
- /*
- * The buffer_locked() || buffer_dirty() tests here are simply an
- * optimisation tweak. If anyone else in the system decides to
- * lock this buffer later on, we'll blow up. There doesn't seem
- * to be a good reason why they should do this.
- */
- if (jh->b_cp_transaction &&
- (buffer_locked(jh2bh(jh)) || buffer_dirty(jh2bh(jh)))) {
+ locked = test_and_set_bit(BH_Lock, &bh->b_state);
+ if (locked) {
+ /* We can't reliably test the buffer state if we found
+ * it already locked, so just wait for the lock and
+ * retry. */
unlock_journal(journal);
- lock_buffer(jh2bh(jh));
- spin_lock(&journal_datalist_lock);
- if (jh->b_cp_transaction && buffer_dirty(jh2bh(jh))) {
- /* OK, we need to steal it */
- JBUFFER_TRACE(jh, "stealing from checkpoint mode");
- J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
- J_ASSERT_JH(jh, jh->b_frozen_data == NULL);
-
- J_ASSERT(handle->h_buffer_credits > 0);
- handle->h_buffer_credits--;
-
- /* This will clear BH_Dirty and set BH_JBDDirty. */
- JBUFFER_TRACE(jh, "file as BJ_Reserved");
- __journal_file_buffer(jh, transaction, BJ_Reserved);
-
- /* And pull it off BUF_DIRTY, onto BUF_CLEAN */
- refile_buffer(jh2bh(jh));
+ __wait_on_buffer(bh);
+ lock_journal(journal);
+ goto repeat;
+ }
+
+ /* We now hold the buffer lock so it is safe to query the buffer
+ * state. Is the buffer dirty?
+ *
+ * If so, there are two possibilities. The buffer may be
+ * non-journaled, and undergoing a quite legitimate writeback.
+ * Otherwise, it is journaled, and we don't expect dirty buffers
+ * in that state (the buffers should be marked JBD_Dirty
+ * instead.) So either the IO is being done under our own
+ * control and this is a bug, or it's a third party IO such as
+ * dump(8) (which may leave the buffer scheduled for read ---
+ * ie. locked but not dirty) or tune2fs (which may actually have
+ * the buffer dirtied, ugh.) */

- /*
- * The buffer is now hidden from bdflush. It is
- * metadata against the current transaction.
- */
- JBUFFER_TRACE(jh, "steal from cp mode is complete");
+ if (buffer_dirty(bh)) {
+ spin_lock(&journal_datalist_lock);
+ /* First question: is this buffer already part of the
+ * current transaction or the existing committing
+ * transaction? */
+ if (jh->b_transaction) {
+ J_ASSERT_JH(jh, jh->b_transaction == transaction ||
+ jh->b_transaction == journal->j_committing_transaction);
+ if (jh->b_next_transaction)
+ J_ASSERT_JH(jh, jh->b_next_transaction == transaction);
+ JBUFFER_TRACE(jh, "Unexpected dirty buffer");
+ jbd_unexpected_dirty_buffer(jh);
}
spin_unlock(&journal_datalist_lock);
- unlock_buffer(jh2bh(jh));
- lock_journal(journal);
- goto repeat;
}

- J_ASSERT_JH(jh, !buffer_locked(jh2bh(jh)));
+ unlock_buffer(bh);

error = -EROFS;
if (is_handle_aborted(handle))
@@ -1978,6 +1969,10 @@
__blist_add_buffer(list, jh);
jh->b_jlist = jlist;

+ /* The following list of buffer states needs to be consistent
+ * with __jbd_unexpected_dirty_buffer()'s handling of dirty
+ * state. */
+
if (jlist == BJ_Metadata || jlist == BJ_Reserved ||
jlist == BJ_Shadow || jlist == BJ_Forget) {
if (atomic_set_buffer_clean(jh2bh(jh))) {
--- linux-ext3-2.4merge-2/fs/jbd/journal.c.=K0000=.orig Thu Aug 22 22:53:04 2002
+++ linux-ext3-2.4merge-2/fs/jbd/journal.c Thu Aug 22 23:43:16 2002
@@ -1485,6 +1485,49 @@
unlock_journal(journal);
}

+
+/*
+ * Report any unexpected dirty buffers which turn up. Normally those
+ * indicate an error, but they can occur if the user is running (say)
+ * tune2fs to modify the live filesystem, so we need the option of
+ * continuing as gracefully as possible. #
+ *
+ * The caller should already hold the journal lock and
+ * journal_datalist_lock spinlock: most callers will need those anyway
+ * in order to probe the buffer's journaling state safely.
+ */
+void __jbd_unexpected_dirty_buffer(char *function, int line,
+ struct journal_head *jh)
+{
+ struct buffer_head *bh = jh2bh(jh);
+ int jlist;
+
+ if (buffer_dirty(bh)) {
+ printk ("%sUnexpected dirty buffer encountered at "
+ "%s:%d (%s blocknr %lu)\n",
+ KERN_WARNING, function, line,
+ kdevname(bh->b_dev), bh->b_blocknr);
+#ifdef JBD_PARANOID_WRITES
+ J_ASSERT_BH (bh, !buffer_dirty(bh));
+#endif
+
+ /* If this buffer is one which might reasonably be dirty
+ * --- ie. data, or not part of this journal --- then
+ * we're OK to leave it alone, but otherwise we need to
+ * move the dirty bit to the journal's own internal
+ * JBDDirty bit. */
+ jlist = jh->b_jlist;
+
+ if (jlist == BJ_Metadata || jlist == BJ_Reserved ||
+ jlist == BJ_Shadow || jlist == BJ_Forget) {
+ if (atomic_set_buffer_clean(jh2bh(jh))) {
+ set_bit(BH_JBDDirty, &jh2bh(jh)->b_state);
+ }
+ }
+ }
+}
+
+
int journal_blocks_per_page(struct inode *inode)
{
return 1 << (PAGE_CACHE_SHIFT - inode->i_sb->s_blocksize_bits);
-
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