jbd: fix the way the b_modified flag is cleared (5bc833fe) · Commits · 戴 / test

Currently at the start of a journal commit we loop through all of the buffers on the committing transaction and clear the b_modified flag (the flag that is set when a transaction modifies the buffer) under the j_list_lock. The problem is that everywhere else this flag is modified only under the jbd lock buffer flag, so it will race with a running transaction who could potentially set it, and have it unset by the committing transaction. This is also a big waste, you can have several thousands of buffers that you are clearing the modified flag on when you may not need to. This patch removes this code and instead clears the b_modified flag upon entering do_get_write_access/journal_get_create_access, so if that transaction does indeed use the buffer then it will be accounted for properly, and if it does not then we know we didn't use it. That will be important for the next patch in this series. Tested thoroughly by myself using postmark/iozone/bonnie++. Signed-off-by:

Josef Bacik <jbacik@redhat.com> Cc: <linux-ext4@vger.kernel.org> Acked-by:

Jan Kara <jack@ucw.cz> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

fs/jbd/commit.c

+0 −16

Original line number	Diff line number	Diff line
		@@ -406,22 +406,6 @@ void journal_commit_transaction(journal_t *journal)

		jbd_debug (3, "JBD: commit phase 2\n");

		/*
		* First, drop modified flag: all accesses to the buffers
		* will be tracked for a new trasaction only -bzzz
		*/
		spin_lock(&journal->j_list_lock);
		if (commit_transaction->t_buffers) {
		new_jh = jh = commit_transaction->t_buffers->b_tnext;
		do {
		J_ASSERT_JH(new_jh, new_jh->b_modified == 1 \|\|
		new_jh->b_modified == 0);
		new_jh->b_modified = 0;
		new_jh = new_jh->b_tnext;
		} while (new_jh != jh);
		}
		spin_unlock(&journal->j_list_lock);

		/*
		* Now start flushing things to disk, in the order they appear
		* on the transaction lists. Data blocks go first.

fs/jbd/transaction.c

+13 −0

Original line number	Diff line number	Diff line
		@@ -608,6 +608,12 @@ repeat:
		jh->b_next_transaction == transaction)
		goto done;

		/*
		* this is the first time this transaction is touching this buffer,
		* reset the modified flag
		*/
		jh->b_modified = 0;

		/*
		* If there is already a copy-out version of this buffer, then we don't
		* need to make another one
		@@ -820,9 +826,16 @@ int journal_get_create_access(handle_t handle, struct buffer_head bh)

		if (jh->b_transaction == NULL) {
		jh->b_transaction = transaction;

		/* first access by this transaction */
		jh->b_modified = 0;

		JBUFFER_TRACE(jh, "file as BJ_Reserved");
		__journal_file_buffer(jh, transaction, BJ_Reserved);
		} else if (jh->b_transaction == journal->j_committing_transaction) {
		/* first access by this transaction */
		jh->b_modified = 0;

		JBUFFER_TRACE(jh, "set next transaction");
		jh->b_next_transaction = transaction;
		}

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