Date:	Tue, 11 Nov 2014 23:07:07 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:	Theodore Ts'o <tytso@....edu>
Subject: [PATCH,RFC] ext4: add lazytime mount option

Add a new mount option which enables a new "lazytime" mode.  This mode
causes atime, mtime, and ctime updates to only be made to the
in-memory version of the inode.  The on-disk times will only get
updated when (a) when the inode table block for the inode needs to be
updated for some non-time related change involving any inode in the
block, (b) if userspace calls fsync(), or (c) the refcount on an
undeleted inode goes to zero (in most cases, when the last file
descriptor assoicated with the inode is closed).

This is legal according to POSIX because there are no guarantees after
a crash unless userspace explicitly requests via a fsync(2) call.  So
in fact, this a better way of reducing the disk traffic resulting from
atime is use lazytime instead of relatime or noatime.  Enabling
lazytime and disabling the default realtime will result in fewer extra
disk writes, and has the benefits of being POSIX-compliant --- since
either noatime and relatime violates POSIX.

The lazytime mode reduces pressure on the journal spinlocks, since
time updates resulting from calls to file_update_time() are almost
always done using separate jbd2 handles.  For allocating writes, the
inode will need to be updated anyway when i_blocks change, and so the
mtime updates will be folded into jbd2 handle in the ext4 write path.

In addition, for workloads feature a large number of random write to a
preallocated file, the lazytime mount option significantly reduces
writes to the inode table.  The repeated 4k writes to a single block
will result in undesirable stress on flash devices and SMR disk
drives.  Even on conventional HDD's, the repeated writes to the inode
table block will trigger Adjacent Track Interference (ATI) remediation
latencies, which very negatively impact 99.9 percentile latencies ---
which is a very big deal for web serving tiers (for example).

n.b.: because of the many wins of this mode, we may want to enable
lazytime updates by default in the future.  If you know of use cases
where having inaccurate mtime values after a crash would be extremely
problematic, please us know at linux-ext4@...r.kernel.org.

Google-Bug-Id: 18297052

Signed-off-by: Theodore Ts'o <tytso@....edu>
---
 fs/ext4/ext4.h     |  3 +++
 fs/ext4/file.c     |  1 +
 fs/ext4/fsync.c    |  3 +++
 fs/ext4/inode.c    | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
 fs/ext4/namei.c    |  2 ++
 fs/ext4/super.c    | 14 ++++++++++++
 fs/ext4/symlink.c  |  2 ++
 fs/inode.c         | 36 +++++++++++++++++++++++++++++++
 include/linux/fs.h |  2 ++
 9 files changed, 124 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index c55a1fa..494c504 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -970,6 +970,7 @@ struct ext4_inode_info {
 #define EXT4_MOUNT_ERRORS_MASK		0x00070
 #define EXT4_MOUNT_MINIX_DF		0x00080	/* Mimics the Minix statfs */
 #define EXT4_MOUNT_NOLOAD		0x00100	/* Don't use existing journal*/
+#define EXT4_MOUNT_LAZYTIME		0x00200 /* Update the time lazily */
 #define EXT4_MOUNT_DATA_FLAGS		0x00C00	/* Mode for data writes: */
 #define EXT4_MOUNT_JOURNAL_DATA		0x00400	/* Write data to journal */
 #define EXT4_MOUNT_ORDERED_DATA		0x00800	/* Flush data before commit */
@@ -1407,6 +1408,7 @@ enum {
 	EXT4_STATE_MAY_INLINE_DATA,	/* may have in-inode data */
 	EXT4_STATE_ORDERED_MODE,	/* data=ordered mode */
 	EXT4_STATE_EXT_PRECACHED,	/* extents have been precached */
+	EXT4_STATE_DIRTY_TIME,		/* the time needs to be updated */
 };
 
 #define EXT4_INODE_BIT_FNS(name, field, offset)				\
@@ -2114,6 +2116,7 @@ extern int  ext4_write_inode(struct inode *, struct writeback_control *);
 extern int  ext4_setattr(struct dentry *, struct iattr *);
 extern int  ext4_getattr(struct vfsmount *mnt, struct dentry *dentry,
 				struct kstat *stat);
+extern int ext4_update_time(struct inode *, struct timespec *, int);
 extern void ext4_evict_inode(struct inode *);
 extern void ext4_clear_inode(struct inode *);
 extern int  ext4_sync_inode(handle_t *, struct inode *);
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 8131be8..2cf6aaf 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -603,6 +603,7 @@ const struct file_operations ext4_file_operations = {
 const struct inode_operations ext4_file_inode_operations = {
 	.setattr	= ext4_setattr,
 	.getattr	= ext4_getattr,
+	.update_time	= ext4_update_time,
 	.setxattr	= generic_setxattr,
 	.getxattr	= generic_getxattr,
 	.listxattr	= ext4_listxattr,
diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
index a8bc47f..ba05c83 100644
--- a/fs/ext4/fsync.c
+++ b/fs/ext4/fsync.c
@@ -116,6 +116,9 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)
 	ret = filemap_write_and_wait_range(inode->i_mapping, start, end);
 	if (ret)
 		return ret;
+
+	if (!datasync && ext4_test_inode_state(inode, EXT4_STATE_DIRTY_TIME))
+		ext4_dirty_inode(inode, 0);
 	/*
 	 * data=writeback,ordered:
 	 *  The caller's filemap_fdatawrite()/wait will sync the data.
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 3356ab5..1b5e4bd 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4163,6 +4163,46 @@ static int ext4_inode_blocks_set(handle_t *handle,
 }
 
 /*
+ * Opportunistically update the other time fields for other inodes in
+ * the same inode table block.
+ */
+static void ext4_update_other_inodes_time(struct super_block *sb,
+					  unsigned long orig_ino, char *buf)
+{
+	struct ext4_inode_info	*ei;
+	struct ext4_inode	*raw_inode;
+	unsigned long		ino;
+	struct inode		*inode;
+	int		i, inodes_per_block = EXT4_SB(sb)->s_inodes_per_block;
+	int		inode_size = EXT4_INODE_SIZE(sb);
+
+	ino = orig_ino & ~(inodes_per_block - 1);
+	for (i = 0; i < inodes_per_block; i++, ino++, buf += inode_size) {
+		if (ino == orig_ino)
+			continue;
+		inode = find_active_inode_nowait(sb, ino);
+		if (!inode ||
+		    !ext4_test_inode_state(inode, EXT4_STATE_DIRTY_TIME)) {
+			iput(inode);
+			continue;
+		}
+		raw_inode = (struct ext4_inode *) buf;
+		ei = EXT4_I(inode);
+
+		smp_mb__before_spinlock();
+		spin_lock(&ei->i_raw_lock);
+		ext4_clear_inode_state(inode, EXT4_STATE_DIRTY_TIME);
+		EXT4_INODE_GET_XTIME(i_ctime, inode, raw_inode);
+		EXT4_INODE_GET_XTIME(i_mtime, inode, raw_inode);
+		EXT4_INODE_GET_XTIME(i_atime, inode, raw_inode);
+		EXT4_EINODE_GET_XTIME(i_crtime, ei, raw_inode);
+		ext4_inode_csum_set(inode, raw_inode, ei);
+		spin_unlock(&ei->i_raw_lock);
+		iput(inode);
+	}
+}
+
+/*
  * Post the struct inode info into an on-disk inode location in the
  * buffer-cache.  This gobbles the caller's reference to the
  * buffer_head in the inode location struct.
@@ -4182,7 +4222,9 @@ static int ext4_do_update_inode(handle_t *handle,
 	uid_t i_uid;
 	gid_t i_gid;
 
+	smp_mb__before_spinlock();
 	spin_lock(&ei->i_raw_lock);
+	ext4_clear_inode_state(inode, EXT4_STATE_DIRTY_TIME);
 
 	/* For fields not tracked in the in-memory inode,
 	 * initialise them to zero for new inodes. */
@@ -4273,8 +4315,8 @@ static int ext4_do_update_inode(handle_t *handle,
 	}
 
 	ext4_inode_csum_set(inode, raw_inode, ei);
-
 	spin_unlock(&ei->i_raw_lock);
+	ext4_update_other_inodes_time(inode->i_sb, inode->i_ino, bh->b_data);
 
 	BUFFER_TRACE(bh, "call ext4_handle_dirty_metadata");
 	rc = ext4_handle_dirty_metadata(handle, NULL, bh);
@@ -4622,6 +4664,24 @@ int ext4_getattr(struct vfsmount *mnt, struct dentry *dentry,
 	return 0;
 }
 
+int ext4_update_time(struct inode *inode, struct timespec *time, int flags)
+{
+	if (flags & S_ATIME)
+		inode->i_atime = *time;
+	if (flags & S_VERSION)
+		inode_inc_iversion(inode);
+	if (flags & S_CTIME)
+		inode->i_ctime = *time;
+	if (flags & S_MTIME)
+		inode->i_mtime = *time;
+	if (test_opt(inode->i_sb, LAZYTIME)) {
+		smp_wmb();
+		ext4_set_inode_state(inode, EXT4_STATE_DIRTY_TIME);
+	} else
+		mark_inode_dirty_sync(inode);
+	return 0;
+}
+
 static int ext4_index_trans_blocks(struct inode *inode, int lblocks,
 				   int pextents)
 {
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 4262118..f782040 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -3532,6 +3532,7 @@ const struct inode_operations ext4_dir_inode_operations = {
 	.tmpfile	= ext4_tmpfile,
 	.rename2	= ext4_rename2,
 	.setattr	= ext4_setattr,
+	.update_time	= ext4_update_time,
 	.setxattr	= generic_setxattr,
 	.getxattr	= generic_getxattr,
 	.listxattr	= ext4_listxattr,
@@ -3545,6 +3546,7 @@ const struct inode_operations ext4_special_inode_operations = {
 	.setattr	= ext4_setattr,
 	.setxattr	= generic_setxattr,
 	.getxattr	= generic_getxattr,
+	.update_time	= ext4_update_time,
 	.listxattr	= ext4_listxattr,
 	.removexattr	= generic_removexattr,
 	.get_acl	= ext4_get_acl,
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 2c9e686..16c9983 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -910,6 +910,14 @@ static int ext4_drop_inode(struct inode *inode)
 	int drop = generic_drop_inode(inode);
 
 	trace_ext4_drop_inode(inode, drop);
+	if (!drop && ext4_test_inode_state(inode, EXT4_STATE_DIRTY_TIME)) {
+		atomic_inc(&inode->i_count);
+		spin_unlock(&inode->i_lock);
+		ext4_dirty_inode(inode, 0);
+		spin_lock(&inode->i_lock);
+		if (atomic_dec_and_test(&inode->i_count))
+			drop = generic_drop_inode(inode);
+	}
 	return drop;
 }
 
@@ -1142,6 +1150,7 @@ enum {
 	Opt_noquota, Opt_barrier, Opt_nobarrier, Opt_err,
 	Opt_usrquota, Opt_grpquota, Opt_i_version,
 	Opt_stripe, Opt_delalloc, Opt_nodelalloc, Opt_mblk_io_submit,
+	Opt_lazytime, Opt_nolazytime,
 	Opt_nomblk_io_submit, Opt_block_validity, Opt_noblock_validity,
 	Opt_inode_readahead_blks, Opt_journal_ioprio,
 	Opt_dioread_nolock, Opt_dioread_lock,
@@ -1204,6 +1213,8 @@ static const match_table_t tokens = {
 	{Opt_i_version, "i_version"},
 	{Opt_stripe, "stripe=%u"},
 	{Opt_delalloc, "delalloc"},
+	{Opt_lazytime, "lazytime"},
+	{Opt_nolazytime, "nolazytime"},
 	{Opt_nodelalloc, "nodelalloc"},
 	{Opt_removed, "mblk_io_submit"},
 	{Opt_removed, "nomblk_io_submit"},
@@ -1361,6 +1372,8 @@ static const struct mount_opts {
 	 MOPT_EXT4_ONLY | MOPT_SET | MOPT_EXPLICIT},
 	{Opt_nodelalloc, EXT4_MOUNT_DELALLOC,
 	 MOPT_EXT4_ONLY | MOPT_CLEAR},
+	{Opt_lazytime, EXT4_MOUNT_LAZYTIME, MOPT_SET},
+	{Opt_nolazytime, EXT4_MOUNT_LAZYTIME, MOPT_CLEAR},
 	{Opt_journal_checksum, EXT4_MOUNT_JOURNAL_CHECKSUM,
 	 MOPT_EXT4_ONLY | MOPT_SET},
 	{Opt_journal_async_commit, (EXT4_MOUNT_JOURNAL_ASYNC_COMMIT |
@@ -3514,6 +3527,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 
 	/* Set defaults before we parse the mount options */
 	def_mount_opts = le32_to_cpu(es->s_default_mount_opts);
+	set_opt(sb, LAZYTIME);
 	set_opt(sb, INIT_INODE_TABLE);
 	if (def_mount_opts & EXT4_DEFM_DEBUG)
 		set_opt(sb, DEBUG);
diff --git a/fs/ext4/symlink.c b/fs/ext4/symlink.c
index ff37119..7c92b93 100644
--- a/fs/ext4/symlink.c
+++ b/fs/ext4/symlink.c
@@ -35,6 +35,7 @@ const struct inode_operations ext4_symlink_inode_operations = {
 	.follow_link	= page_follow_link_light,
 	.put_link	= page_put_link,
 	.setattr	= ext4_setattr,
+	.update_time	= ext4_update_time,
 	.setxattr	= generic_setxattr,
 	.getxattr	= generic_getxattr,
 	.listxattr	= ext4_listxattr,
@@ -45,6 +46,7 @@ const struct inode_operations ext4_fast_symlink_inode_operations = {
 	.readlink	= generic_readlink,
 	.follow_link	= ext4_follow_link,
 	.setattr	= ext4_setattr,
+	.update_time	= ext4_update_time,
 	.setxattr	= generic_setxattr,
 	.getxattr	= generic_getxattr,
 	.listxattr	= ext4_listxattr,
diff --git a/fs/inode.c b/fs/inode.c
index 26753ba..cde073a 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1280,6 +1280,42 @@ struct inode *ilookup(struct super_block *sb, unsigned long ino)
 }
 EXPORT_SYMBOL(ilookup);
 
+/**
+ * find_active_inode_nowait - find an active inode in the inode cache
+ * @sb:		super block of file system to search
+ * @ino:	inode number to search for
+ *
+ * Search for an active inode @ino in the inode cache, and if the
+ * inode is in the cache, the inode is returned with an incremented
+ * reference count.  If the inode is being freed or is newly
+ * initialized, return nothing instead of trying to wait for the inode
+ * initialization or destruction to be complete.
+ */
+struct inode *find_active_inode_nowait(struct super_block *sb,
+				       unsigned long ino)
+{
+	struct hlist_head *head = inode_hashtable + hash(sb, ino);
+	struct inode *inode, *ret_inode = NULL;
+
+	spin_lock(&inode_hash_lock);
+	hlist_for_each_entry(inode, head, i_hash) {
+		if ((inode->i_ino != ino) ||
+		    (inode->i_sb != sb))
+			continue;
+		spin_lock(&inode->i_lock);
+		if ((inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) == 0) {
+			__iget(inode);
+			ret_inode = inode;
+		}
+		spin_unlock(&inode->i_lock);
+		goto out;
+	}
+out:
+	spin_unlock(&inode_hash_lock);
+	return ret_inode;
+}
+EXPORT_SYMBOL(find_active_inode_nowait);
+
 int insert_inode_locked(struct inode *inode)
 {
 	struct super_block *sb = inode->i_sb;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 9ab779e..b5e6b6b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2410,6 +2410,8 @@ extern struct inode *ilookup(struct super_block *sb, unsigned long ino);
 
 extern struct inode * iget5_locked(struct super_block *, unsigned long, int (*test)(struct inode *, void *), int (*set)(struct inode *, void *), void *);
 extern struct inode * iget_locked(struct super_block *, unsigned long);
+extern struct inode *find_active_inode_nowait(struct super_block *,
+					      unsigned long);
 extern int insert_inode_locked4(struct inode *, unsigned long, int (*test)(struct inode *, void *), void *);
 extern int insert_inode_locked(struct inode *);
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
-- 
2.1.0

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