2008-03-01 00:05:16

by Andrew Morton

[permalink] [raw]
Subject: + jbd-fix-the-way-the-b_modified-flag-is-cleared.patch added to -mm tree

The patch titled
jbd: fix the way the b_modified flag is cleared
has been added to the -mm tree. Its filename is

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out what to do about this

The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/

Subject: jbd: fix the way the b_modified flag is cleared
From: Josef Bacik <[email protected]>

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 <[email protected]>
Cc: <[email protected]>
Cc: Jan Kara <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

fs/jbd/commit.c | 16 ----------------
fs/jbd/transaction.c | 13 +++++++++++++
2 files changed, 13 insertions(+), 16 deletions(-)

diff -puN fs/jbd/commit.c~jbd-fix-the-way-the-b_modified-flag-is-cleared fs/jbd/commit.c
--- a/fs/jbd/commit.c~jbd-fix-the-way-the-b_modified-flag-is-cleared
+++ a/fs/jbd/commit.c
@@ -407,22 +407,6 @@ void journal_commit_transaction(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.
diff -puN fs/jbd/transaction.c~jbd-fix-the-way-the-b_modified-flag-is-cleared fs/jbd/transaction.c
--- a/fs/jbd/transaction.c~jbd-fix-the-way-the-b_modified-flag-is-cleared
+++ a/fs/jbd/transaction.c
@@ -609,6 +609,12 @@ repeat:
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 *

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;

Patches currently in -mm which might be from [email protected] are