[PATCH] ext4, jbd2: add an optimized bmap for the journal inode

[PATCH] ext4, jbd2: add an optimized bmap for the journal inode

[Theodore Ts'o]

The generic bmap() function exported by the VFS takes locks and does
checks that are not necessary for the journal inode.  So allow the
file system to set a journal-optimized bmap function in
journal->j_bmap.

The also has the benefit of avoiding some false positives by DEPT.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
 fs/ext4/super.c      | 23 +++++++++++++++++++++++
 fs/jbd2/journal.c    |  9 ++++++---
 include/linux/jbd2.h |  8 ++++++++
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 2192b4111442..46b7345d2b6a 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5742,6 +5742,28 @@ static struct inode *ext4_get_journal_inode(struct super_block *sb,
 	return journal_inode;
 }
 
+static int ext4_journal_bmap(journal_t *journal, sector_t *block)
+{
+	struct ext4_map_blocks map;
+	int ret;
+
+	if (journal->j_inode == NULL)
+		return 0;
+
+	map.m_lblk = *block;
+	map.m_len = 1;
+	ret = ext4_map_blocks(NULL, journal->j_inode, &map, 0);
+	if (ret <= 0) {
+		ext4_msg(journal->j_inode->i_sb, KERN_CRIT,
+			 "journal bmap failed: block %llu ret %d\n",
+			 *block, ret);
+		jbd2_journal_abort(journal, ret ? ret : -EIO);
+		return ret;
+	}
+	*block = map.m_pblk;
+	return 0;
+}
+
 static journal_t *ext4_get_journal(struct super_block *sb,
 				   unsigned int journal_inum)
 {
@@ -5762,6 +5784,7 @@ static journal_t *ext4_get_journal(struct super_block *sb,
 		return NULL;
 	}
 	journal->j_private = sb;
+	journal->j_bmap = ext4_journal_bmap;
 	ext4_init_journal_params(sb, journal);
 	return journal;
 }
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 2696f43e7239..c84f588fdcd0 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -970,10 +970,13 @@ int jbd2_journal_bmap(journal_t *journal, unsigned long blocknr,
 {
 	int err = 0;
 	unsigned long long ret;
-	sector_t block = 0;
+	sector_t block = blocknr;
 
-	if (journal->j_inode) {
-		block = blocknr;
+	if (journal->j_bmap) {
+		err = journal->j_bmap(journal, &block);
+		if (err == 0)
+			*retp = block;
+	} else if (journal->j_inode) {
 		ret = bmap(journal->j_inode, &block);
 
 		if (ret || !block) {
diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 2170e0cc279d..6ffa34c51a11 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1308,6 +1308,14 @@ struct journal_s
 				    struct buffer_head *bh,
 				    enum passtype pass, int off,
 				    tid_t expected_commit_id);
+
+	/**
+	 * @j_bmap:
+	 *
+	 * Bmap function that should be used instead of the generic
+	 * VFS bmap function.
+	 */
+	int (*j_bmap)(struct journal_s *journal, sector_t *block);
 };
 
 #define jbd2_might_wait_for_commit(j) \
-- 
2.31.0

Re: [PATCH] ext4, jbd2: add an optimized bmap for the journal inode

[Eric Biggers]

On Tue, Mar 07, 2023 at 11:16:15PM -0500, Theodore Ts'o wrote:
> The generic bmap() function exported by the VFS takes locks and does
> checks that are not necessary for the journal inode.  So allow the
> file system to set a journal-optimized bmap function in
> journal->j_bmap.
> 
> The also has the benefit of avoiding some false positives by DEPT.
> 
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
> ---
>  fs/ext4/super.c      | 23 +++++++++++++++++++++++
>  fs/jbd2/journal.c    |  9 ++++++---
>  include/linux/jbd2.h |  8 ++++++++
>  3 files changed, 37 insertions(+), 3 deletions(-)
> 

Should this also be marked as a fix for the syzbot report
https://syzkaller.appspot.com/bug?id=e4aaa78795e490421c79f76ec3679006c8ff4cf0 ?

- Eric