[RFC PATCH v2 2/4] ocfs2: switch dio read path from buffer_head to iomap

[Heming Zhao]

This patch migrates the DIO read path from the legacy buffer_head
infrastructure to the modern and more efficient iomap framework.

The rw cluster-lock level is not stashed in iocb->private: iomap DIO owns
that field (it stores a bio there for polled I/O and unconditionally
clears it before calling ->end_io), so any side-channel packed into
iocb->private is corrupted and would trip the end_io lock-state check.
Instead the unlock level is encoded by which iomap_dio_ops is passed -
ocfs2_iomap_dio_ops_pr (PRMODE) or ocfs2_iomap_dio_ops_ex (EXMODE) - and
the *_iter() functions use the split __iomap_dio_rw() + iomap_dio_complete()
API so they can determine unambiguously, from the return value, whether the
completion handler already dropped the lock (real I/O, sync or -EIOCBQUEUED)
or whether the caller must drop it (no I/O issued, or buffered fallback).

Co-developed-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
Signed-off-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
Signed-off-by: Heming Zhao <heming.zhao@xxxxxxxx>
---
 fs/ocfs2/Kconfig    |   1 +
 fs/ocfs2/aops.c     | 113 ++++++++++++++++++++++++++++++++++++++++++++
 fs/ocfs2/file.c     |  57 ++++++++++++++++++----
 fs/ocfs2/ocfs2.h    |   3 ++
 fs/ocfs2/ocfs2_fs.h |   3 ++
 5 files changed, 167 insertions(+), 10 deletions(-)

diff --git a/fs/ocfs2/Kconfig b/fs/ocfs2/Kconfig
index 2514d36cbe01..bf1678a5eb01 100644
--- a/fs/ocfs2/Kconfig
+++ b/fs/ocfs2/Kconfig
@@ -7,6 +7,7 @@ config OCFS2_FS
 	select CRC32
 	select QUOTA
 	select QUOTA_TREE
+	select FS_IOMAP
 	select FS_POSIX_ACL
 	select LEGACY_DIRECT_IO
 	help
diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
index 08df5e3b5196..12f5f2e3530a 100644
--- a/fs/ocfs2/aops.c
+++ b/fs/ocfs2/aops.c
@@ -4,6 +4,7 @@
  */
 
 #include <linux/fs.h>
+#include <linux/iomap.h>
 #include <linux/slab.h>
 #include <linux/highmem.h>
 #include <linux/pagemap.h>
@@ -2556,6 +2557,118 @@ static ssize_t ocfs2_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 				    ocfs2_dio_end_io, 0);
 }
 
+static int ocfs2_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
+		unsigned int flags, struct iomap *iomap, struct iomap *srcmap)
+{
+	int ret;
+	struct ocfs2_map_block map;
+	struct ocfs2_inode_info *oi = OCFS2_I(inode);
+	u8 blkbits = inode->i_blkbits;
+
+	if ((offset >> blkbits) > OCFS2_MAX_LOGICAL_BLOCK)
+		return -EINVAL;
+
+	/*
+	 * Calculate the first and last logical blocks respectively.
+	 */
+	map.lblk = offset >> blkbits;
+	map.len = min_t(loff_t, (offset + length - 1) >> blkbits,
+			OCFS2_MAX_LOGICAL_BLOCK) - map.lblk + 1;
+	map.flags = 0;
+
+	if (flags & IOMAP_WRITE) {
+		/* todo */
+	} else {
+		down_read(&oi->ip_alloc_sem);
+		ret = ocfs2_map_blocks(inode, &map, 0);
+		up_read(&oi->ip_alloc_sem);
+	}
+
+	if (ret < 0)
+		return ret;
+
+	/*
+	 * Before returning to iomap, let's ensure the allocated mapping
+	 * covers the entire requested length for atomic writes.
+	 */
+	if (flags & IOMAP_ATOMIC) {
+		if (map.len < (length >> blkbits)) {
+			WARN_ON_ONCE(1);
+			return -EINVAL;
+		}
+	}
+
+	iomap->bdev = inode->i_sb->s_bdev;
+	iomap->offset = (u64)map.lblk << blkbits;
+	iomap->length = (u64)map.len << blkbits;
+
+	if (map.pblk == 0) {
+		iomap->type = IOMAP_HOLE;
+		iomap->addr = IOMAP_NULL_ADDR;
+	} else {
+		if (map.flags & OCFS2_MAP_UNWRITTEN) {
+			iomap->type = IOMAP_UNWRITTEN;
+		} else if (map.flags & OCFS2_MAP_MAPPED) {
+			iomap->type = IOMAP_MAPPED;
+		} else {
+			WARN_ON_ONCE(1);
+			return -EIO;
+		}
+		iomap->addr = (sector_t)map.pblk << blkbits;
+	}
+
+	if (map.flags & OCFS2_MAP_NEW)
+		iomap->flags |= IOMAP_F_NEW;
+
+	return 0;
+}
+
+const struct iomap_ops ocfs2_iomap_ops = {
+	.iomap_begin	= ocfs2_iomap_begin,
+};
+
+/*
+ * Direct I/O completion. Like the old ocfs2_dio_end_io(), we use the rw_lock
+ * DLM lock to protect io on one node from truncation on another, and drop it
+ * here once the io has completed. iomap_dio_complete() always calls this,
+ * including on the buffered-fallback (-ENOTBLK) path, so the rw_lock taken in
+ * ocfs2_file_{read,write}_iter() is released exactly once.
+ */
+static int ocfs2_iomap_dio_end_io(struct kiocb *iocb, ssize_t size, int error,
+				  int level)
+{
+	struct inode *inode = file_inode(iocb->ki_filp);
+
+	/*
+	 * iomap_dio_complete() calls us even when no I/O was issued because the
+	 * mapping bounced us back to buffered I/O (reported as size == 0 and
+	 * error == 0). In that case leave the rw_lock held so
+	 * ocfs2_file_{read,write}_iter() keeps it for the buffered retry and
+	 * releases it in its own cleanup path.
+	 */
+	if (size == 0 && error == 0)
+		return 0;
+
+	ocfs2_rw_unlock(inode, level);
+	return error;
+}
+
+static int ocfs2_dio_end_io_r_pr(struct kiocb *iocb, ssize_t size,
+				int error, unsigned int flags)
+{
+	if (error)
+		mlog_ratelimited(ML_ERROR, "Direct IO failed, bytes = %lld errno:%d",
+				 (long long)size, error);
+
+	error = ocfs2_iomap_dio_end_io(iocb, size, error, 0);
+
+bail:
+	return error;
+}
+
+const struct iomap_dio_ops ocfs2_iomap_dio_ops_r_pr = {
+	.end_io = ocfs2_dio_end_io_r_pr,
+};
 const struct address_space_operations ocfs2_aops = {
 	.dirty_folio		= block_dirty_folio,
 	.read_folio		= ocfs2_read_folio,
diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
index d6e977ba6565..a0b3883216bc 100644
--- a/fs/ocfs2/file.c
+++ b/fs/ocfs2/file.c
@@ -9,6 +9,7 @@
 
 #include <linux/capability.h>
 #include <linux/fs.h>
+#include <linux/iomap.h>
 #include <linux/types.h>
 #include <linux/slab.h>
 #include <linux/highmem.h>
@@ -2374,6 +2375,19 @@ static int ocfs2_prepare_inode_for_write(struct file *file,
 	return ret;
 }
 
+static bool ocfs2_should_use_dio(struct kiocb *iocb, struct iov_iter *iter,
+				struct inode *inode)
+{
+	/*
+	 * Fallback to buffered I/O if we see an inode without
+	 * extents.
+	 */
+	if (OCFS2_I(inode)->ip_dyn_features & OCFS2_INLINE_DATA_FL)
+		return false;
+
+	return true;
+}
+
 static ssize_t ocfs2_file_write_iter(struct kiocb *iocb,
 				    struct iov_iter *from)
 {
@@ -2548,7 +2562,6 @@ static ssize_t ocfs2_file_read_iter(struct kiocb *iocb,
 			filp->f_path.dentry->d_name.name,
 			to->nr_segs);	/* GRRRRR */
 
-
 	if (!inode) {
 		ret = -EINVAL;
 		mlog_errno(ret);
@@ -2558,7 +2571,8 @@ static ssize_t ocfs2_file_read_iter(struct kiocb *iocb,
 	if (!direct_io && nowait)
 		return -EOPNOTSUPP;
 
-	ocfs2_iocb_init_rw_locked(iocb);
+	if (!iov_iter_count(to))
+		return 0; /* skip atime */
 
 	/*
 	 * buffered reads protect themselves in ->read_folio().  O_DIRECT reads
@@ -2576,8 +2590,6 @@ static ssize_t ocfs2_file_read_iter(struct kiocb *iocb,
 			goto bail;
 		}
 		rw_level = 0;
-		/* communicate with ocfs2_dio_end_io */
-		ocfs2_iocb_set_rw_locked(iocb, rw_level);
 	}
 
 	/*
@@ -2598,17 +2610,42 @@ static ssize_t ocfs2_file_read_iter(struct kiocb *iocb,
 	}
 	ocfs2_inode_unlock(inode, lock_level);
 
-	ret = generic_file_read_iter(iocb, to);
+	if (direct_io && ocfs2_should_use_dio(iocb, to, inode)) {
+		struct iomap_dio *dio;
+
+		dio = __iomap_dio_rw(iocb, to, &ocfs2_iomap_ops,
+				&ocfs2_iomap_dio_ops_r_pr, 0, NULL, 0);
+		if (dio == NULL) {
+			/* No I/O issued; rw_lock still held. */
+			ret = 0;
+		} else if (IS_ERR(dio)) {
+			ret = PTR_ERR(dio);
+			if (ret == -EIOCBQUEUED)
+				rw_level = -1;
+		} else {
+			ret = iomap_dio_complete(dio);
+			if (ret != 0)
+				rw_level = -1;
+		}
+		/*
+		 * A 0 result means the mapping bounced us back to buffered I/O
+		 * (e.g. inline data); the rw_lock is still held. Clear
+		 * IOCB_DIRECT so generic_file_read_iter() takes the buffered
+		 * path rather than re-entering direct I/O.
+		 */
+		if (ret == 0) {
+			iocb->ki_flags &= ~IOCB_DIRECT;
+			ret = generic_file_read_iter(iocb, to);
+		}
+	} else {
+		iocb->ki_flags &= ~IOCB_DIRECT;
+		ret = generic_file_read_iter(iocb, to);
+	}
 	trace_generic_file_read_iter_ret(ret);
 
 	/* buffered aio wouldn't have proper lock coverage today */
 	BUG_ON(ret == -EIOCBQUEUED && !direct_io);
 
-	/* see ocfs2_file_write_iter */
-	if (ret == -EIOCBQUEUED || !ocfs2_iocb_is_rw_locked(iocb)) {
-		rw_level = -1;
-	}
-
 bail:
 	if (rw_level != -1)
 		ocfs2_rw_unlock(inode, rw_level);
diff --git a/fs/ocfs2/ocfs2.h b/fs/ocfs2/ocfs2.h
index 095f7ae5dded..a775c1869293 100644
--- a/fs/ocfs2/ocfs2.h
+++ b/fs/ocfs2/ocfs2.h
@@ -549,6 +549,9 @@ struct ocfs2_map_block {
 	unsigned int flags;
 };
 
+extern const struct iomap_ops ocfs2_iomap_ops;
+extern const struct iomap_dio_ops ocfs2_iomap_dio_ops_r_pr;
+
 /* Flags used by ocfs2_map_blocks() */
 #define OCFS2_GET_BLOCKS_CREATE	(0x0001)
 
diff --git a/fs/ocfs2/ocfs2_fs.h b/fs/ocfs2/ocfs2_fs.h
index c501eb3cdcda..000bd014acbd 100644
--- a/fs/ocfs2/ocfs2_fs.h
+++ b/fs/ocfs2/ocfs2_fs.h
@@ -314,6 +314,9 @@
  */
 #define OCFS2_CLUSTER_O2CB_GLOBAL_HEARTBEAT	(0x01)
 
+/* Max logical block we can support */
+#define OCFS2_MAX_LOGICAL_BLOCK	(0xFFFFFFFE)
+
 struct ocfs2_system_inode_info {
 	char	*si_name;
 	int	si_iflags;
-- 
2.54.0