ext4: introduce seq counter for the extent status entry

In the iomap_write_iter(), the iomap buffered write frame does not hold
any locks between querying the inode extent mapping info and performing
page cache writes. As a result, the extent mapping can be changed due to
concurrent I/O in flight. Similarly, in the iomap_writepage_map(), the
write-back process faces a similar problem: concurrent changes can
invalidate the extent mapping before the I/O is submitted.

Therefore, both of these processes must recheck the mapping info after
acquiring the folio lock. To address this, similar to XFS, we propose
introducing an extent sequence number to serve as a validity cookie for
the extent. After commit 24b7a2331fcd ("ext4: clairfy the rules for
modifying extents"), we can ensure the extent information should always
be processed through the extent status tree, and the extent status tree
is always uptodate under i_rwsem or invalidate_lock or folio lock, so
it's safe to introduce this sequence number. The sequence number will be
increased whenever the extent status tree changes, preparing for the
buffered write iomap conversion.

Besides, this mechanism is also applicable for the moving extents case.
In move_extent_per_page(), it also needs to reacquire data_sem and check
the mapping info again under the folio lock.

Signed-off-by: Zhang Yi <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Theodore Ts'o <[e-mail]>

fs/ext4/ext4.h

···
 	ext4_lblk_t i_es_shrink_lblk;	/* Offset where we start searching for
 					   extents to shrink. Protected by
 					   i_es_lock  */
+	u64 i_es_seq;			/* Change counter for extents.
+					   Protected by i_es_lock */
 
 	/* ialloc */
 	ext4_group_t	i_last_alloc_group;

fs/ext4/extents_status.c

···
 	return es->es_lblk + es->es_len - 1;
 }
 
+static inline void ext4_es_inc_seq(struct inode *inode)
+{
+	struct ext4_inode_info *ei = EXT4_I(inode);
+
+	WRITE_ONCE(ei->i_es_seq, ei->i_es_seq + 1);
+}
+
 /*
  * search through the tree for an delayed extent with a given offset.  If
  * it can't be found, try to find next extent.
···
 	newes.es_lblk = lblk;
 	newes.es_len = len;
 	ext4_es_store_pblock_status(&newes, pblk, status);
-	trace_ext4_es_insert_extent(inode, &newes);
 
 	ext4_es_insert_extent_check(inode, &newes);
 
···
 		}
 		pending = err3;
 	}
+	/*
+	 * TODO: For cache on-disk extents, there is no need to increment
+	 * the sequence counter, this requires future optimization.
+	 */
+	ext4_es_inc_seq(inode);
 error:
 	write_unlock(&EXT4_I(inode)->i_es_lock);
 	/*
···
 	if (err1 || err2 || err3 < 0)
 		goto retry;
 
+	trace_ext4_es_insert_extent(inode, &newes);
 	ext4_es_print_tree(inode);
 	return;
 }
···
 	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
 		return;
 
-	trace_ext4_es_remove_extent(inode, lblk, len);
 	es_debug("remove [%u/%u) from extent status tree of inode %lu\n",
 		 lblk, len, inode->i_ino);
 
···
 	 */
 	write_lock(&EXT4_I(inode)->i_es_lock);
 	err = __es_remove_extent(inode, lblk, end, &reserved, es);
+	if (err)
+		goto error;
 	/* Free preallocated extent if it didn't get used. */
 	if (es) {
 		if (!es->es_len)
 			__es_free_extent(es);
 		es = NULL;
 	}
+	ext4_es_inc_seq(inode);
+error:
 	write_unlock(&EXT4_I(inode)->i_es_lock);
 	if (err)
 		goto retry;
 
+	trace_ext4_es_remove_extent(inode, lblk, len);
 	ext4_es_print_tree(inode);
 	ext4_da_release_space(inode, reserved);
 }
···
 	newes.es_lblk = lblk;
 	newes.es_len = len;
 	ext4_es_store_pblock_status(&newes, ~0, EXTENT_STATUS_DELAYED);
-	trace_ext4_es_insert_delayed_extent(inode, &newes, lclu_allocated,
-					    end_allocated);
 
 	ext4_es_insert_extent_check(inode, &newes);
 
···
 			pr2 = NULL;
 		}
 	}
+	ext4_es_inc_seq(inode);
 error:
 	write_unlock(&EXT4_I(inode)->i_es_lock);
 	if (err1 || err2 || err3 < 0)
 		goto retry;
 
+	trace_ext4_es_insert_delayed_extent(inode, &newes, lclu_allocated,
+					    end_allocated);
 	ext4_es_print_tree(inode);
 	ext4_print_pending_tree(inode);
 	return;

fs/ext4/super.c

···
 	ei->i_es_all_nr = 0;
 	ei->i_es_shk_nr = 0;
 	ei->i_es_shrink_lblk = 0;
+	ei->i_es_seq = 0;
 	ei->i_reserved_data_blocks = 0;
 	spin_lock_init(&(ei->i_block_reservation_lock));
 	ext4_init_pending_tree(&ei->i_pending_tree);

include/trace/events/ext4.h

···
 		__field(	ext4_lblk_t,	lblk		)
 		__field(	ext4_lblk_t,	len		)
 		__field(	ext4_fsblk_t,	pblk		)
-		__field(	char, status	)
+		__field(	char,		status		)
+		__field(	u64,		seq		)
 	),
 
 	TP_fast_assign(
···
 		__entry->len	= es->es_len;
 		__entry->pblk	= ext4_es_show_pblock(es);
 		__entry->status	= ext4_es_status(es);
+		__entry->seq	= EXT4_I(inode)->i_es_seq;
 	),
 
-	TP_printk("dev %d,%d ino %lu es [%u/%u) mapped %llu status %s",
+	TP_printk("dev %d,%d ino %lu es [%u/%u) mapped %llu status %s seq %llu",
 		  MAJOR(__entry->dev), MINOR(__entry->dev),
 		  (unsigned long) __entry->ino,
 		  __entry->lblk, __entry->len,
-		  __entry->pblk, show_extent_status(__entry->status))
+		  __entry->pblk, show_extent_status(__entry->status),
+		  __entry->seq)
 );
 
 DEFINE_EVENT(ext4__es_extent, ext4_es_insert_extent,
···
 		__field(	ino_t,	ino			)
 		__field(	loff_t,	lblk			)
 		__field(	loff_t,	len			)
+		__field(	u64,	seq			)
 	),
 
 	TP_fast_assign(
···
 		__entry->ino	= inode->i_ino;
 		__entry->lblk	= lblk;
 		__entry->len	= len;
+		__entry->seq	= EXT4_I(inode)->i_es_seq;
 	),
 
-	TP_printk("dev %d,%d ino %lu es [%lld/%lld)",
+	TP_printk("dev %d,%d ino %lu es [%lld/%lld) seq %llu",
 		  MAJOR(__entry->dev), MINOR(__entry->dev),
 		  (unsigned long) __entry->ino,
-		  __entry->lblk, __entry->len)
+		  __entry->lblk, __entry->len, __entry->seq)
 );
 
 TRACE_EVENT(ext4_es_find_extent_range_enter,
···
 		__field(	char,		status		)
 		__field(	bool,		lclu_allocated	)
 		__field(	bool,		end_allocated	)
+		__field(	u64,		seq		)
 	),
 
 	TP_fast_assign(
···
 		__entry->status		= ext4_es_status(es);
 		__entry->lclu_allocated	= lclu_allocated;
 		__entry->end_allocated	= end_allocated;
+		__entry->seq		= EXT4_I(inode)->i_es_seq;
 	),
 
-	TP_printk("dev %d,%d ino %lu es [%u/%u) mapped %llu status %s "
-		  "allocated %d %d",
+	TP_printk("dev %d,%d ino %lu es [%u/%u) mapped %llu status %s allocated %d %d seq %llu",
 		  MAJOR(__entry->dev), MINOR(__entry->dev),
 		  (unsigned long) __entry->ino,
 		  __entry->lblk, __entry->len,
 		  __entry->pblk, show_extent_status(__entry->status),
-		  __entry->lclu_allocated, __entry->end_allocated)
+		  __entry->lclu_allocated, __entry->end_allocated,
+		  __entry->seq)
 );
 
 /* fsmap traces */