linux-ext4 - [PATCH] ext4: add rename whiteout support for fast commit

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Message-Id: <20210316221921.1124955-1-harshadshirwadkar@gmail.com>
Date:   Tue, 16 Mar 2021 15:19:21 -0700
From:   Harshad Shirwadkar <harshadshirwadkar@...il.com>
To:     linux-ext4@...r.kernel.org
Cc:     amir73il@...il.com, tytso@....edu,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>
Subject: [PATCH] ext4: add rename whiteout support for fast commit

This patch adds rename whiteout support in fast commits. Note that the
whiteout object that gets created is actually char device. Which
imples, the function ext4_inode_journal_mode(struct inode *inode)
would return "JOURNAL_DATA" for this inode. This has a consequence in
fast commit code that it will make creation of the whiteout object a
fast-commit ineligible behavior and thus will fall back to full
commits. With this patch, this can be observed by running fast commits
with rename whiteout and seeing the stats generated by ext4_fc_stats
tracepoint as follows:

ext4_fc_stats: dev 254:32 fc ineligible reasons:
XATTR:0, CROSS_RENAME:0, JOURNAL_FLAG_CHANGE:0, NO_MEM:0, SWAP_BOOT:0,
RESIZE:0, RENAME_DIR:0, FALLOC_RANGE:0, INODE_JOURNAL_DATA:16;
num_commits:6, ineligible: 6, numblks: 3

So in short, this patch guarantees that in case of rename whiteout, we
fall back to full commits.

Amir mentioned that instead of creating a new whiteout object for
every rename, we can create a static whiteout object with irrelevant
nlink. That will make fast commits to not fall back to full
commit. But until this happens, this patch will ensure correctness by
falling back to full commits.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>
---
 fs/ext4/ext4.h        | 2 ++
 fs/ext4/fast_commit.c | 9 +++++++--
 fs/ext4/namei.c       | 3 +++
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index f6a36a0e07c1..d7039e86d705 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -2807,6 +2807,8 @@ void __ext4_fc_track_link(handle_t *handle, struct inode *inode,
 	struct dentry *dentry);
 void ext4_fc_track_unlink(handle_t *handle, struct dentry *dentry);
 void ext4_fc_track_link(handle_t *handle, struct dentry *dentry);
+void __ext4_fc_track_create(handle_t *handle, struct inode *inode,
+			    struct dentry *dentry);
 void ext4_fc_track_create(handle_t *handle, struct dentry *dentry);
 void ext4_fc_track_inode(handle_t *handle, struct inode *inode);
 void ext4_fc_mark_ineligible(struct super_block *sb, int reason);
diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index 619412134bbf..d5c4aaa69665 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -513,10 +513,10 @@ void ext4_fc_track_link(handle_t *handle, struct dentry *dentry)
 	__ext4_fc_track_link(handle, d_inode(dentry), dentry);
 }
 
-void ext4_fc_track_create(handle_t *handle, struct dentry *dentry)
+void __ext4_fc_track_create(handle_t *handle, struct inode *inode,
+			  struct dentry *dentry)
 {
 	struct __track_dentry_update_args args;
-	struct inode *inode = d_inode(dentry);
 	int ret;
 
 	args.dentry = dentry;
@@ -527,6 +527,11 @@ void ext4_fc_track_create(handle_t *handle, struct dentry *dentry)
 	trace_ext4_fc_track_create(inode, dentry, ret);
 }
 
+void ext4_fc_track_create(handle_t *handle, struct dentry *dentry)
+{
+	__ext4_fc_track_create(handle, d_inode(dentry), dentry);
+}
+
 /* __track_fn for inode tracking */
 static int __track_inode(struct inode *inode, void *arg, bool update)
 {
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 115762180801..38176c36dda5 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -3846,6 +3846,7 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 		retval = ext4_mark_inode_dirty(handle, whiteout);
 		if (unlikely(retval))
 			goto end_rename;
+
 	}
 	if (!new.bh) {
 		retval = ext4_add_entry(handle, new.dentry, old.inode);
@@ -3919,6 +3920,8 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 			ext4_fc_track_unlink(handle, new.dentry);
 		__ext4_fc_track_link(handle, old.inode, new.dentry);
 		__ext4_fc_track_unlink(handle, old.inode, old.dentry);
+		if (whiteout)
+			__ext4_fc_track_create(handle, whiteout, old.dentry);
 	}
 
 	if (new.inode) {
-- 
2.31.0.rc2.261.g7f71774620-goog