[RFC PATCH 1/3] ocfs2: Add new ocfs2_map_blocks() to introduce iomap feature

[Heming Zhao]

As part of migrating OCFS2 DIO read/write code paths towards the modern
and high-performant iomap framework, this patch introduces an iomap API
to replace old high-overhead VFS buffer_head structure paths.

This patch establishes the foundational block mapping routines required by
subsequent iomap integration patches. The implementation draws
inspiration from ext4_map_blocks().

Signed-off-by: Heming Zhao <heming.zhao@xxxxxxxx>
---
 fs/ocfs2/aops.c           | 94 +++++++++++++++++++++++++++++++++++++++
 fs/ocfs2/aops.h           |  2 +
 fs/ocfs2/buffer_head_io.c | 12 -----
 fs/ocfs2/ocfs2.h          | 45 ++++++++++++++++++-
 4 files changed, 140 insertions(+), 13 deletions(-)

diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
index 4acdbb70882c..08df5e3b5196 100644
--- a/fs/ocfs2/aops.c
+++ b/fs/ocfs2/aops.c
@@ -126,6 +126,100 @@ static int ocfs2_lock_get_block(struct inode *inode, sector_t iblock,
 	return ret;
 }
 
+int ocfs2_map_blocks(struct inode *inode, struct ocfs2_map_block *map,
+		    int flags)
+{
+	int err = 0;
+	unsigned int ext_flags;
+	u64 max_blocks = map->len;
+	u64 p_blkno, count, past_eof;
+	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb);
+	int create = flags & OCFS2_GET_BLOCKS_CREATE;
+
+	if (OCFS2_I(inode)->ip_flags & OCFS2_INODE_SYSTEM_FILE)
+		mlog(ML_NOTICE, "map_block on system inode 0x%p (%llu)\n",
+		     inode, inode->i_ino);
+
+	if (S_ISLNK(inode->i_mode)) {
+		/*
+		 * TODO: refer ocfs2_get_block() to handle
+		 * ocfs2_read_folio in the future
+		 */
+		mlog(ML_NOTICE, "map_block on S_ISLNK file, node 0x%p (%llu)\n",
+		     inode, inode->i_ino);
+		dump_stack();
+		goto bail;
+	}
+
+	err = ocfs2_extent_map_get_blocks(inode, map->lblk, &p_blkno, &count,
+					  &ext_flags);
+	if (err) {
+		mlog(ML_ERROR, "get_blocks() failed, inode: 0x%p, "
+		     "block: %llu\n", inode, map->lblk);
+		goto bail;
+	}
+
+	if (max_blocks < count)
+		count = max_blocks;
+
+	map->pblk = p_blkno;
+	map->len = count;
+
+	/*
+	 * ocfs2 never allocates in this function - the only time we
+	 * need to use MAP_NEW is when we're extending i_size on a file
+	 * system which doesn't support holes, in which case MAP_NEW
+	 * allows __block_write_begin() to zero.
+	 *
+	 * If we see this on a sparse file system, then a truncate has
+	 * raced us and removed the cluster. In this case, we clear
+	 * the buffers dirty and uptodate bits and let the buffer code
+	 * ignore it as a hole.
+	 */
+	if (create && map->pblk == 0 && ocfs2_sparse_alloc(osb)) {
+		map->flags &= ~(OCFS2_MAP_DIRTY | OCFS2_MAP_UPTODATE);
+		goto bail;
+	}
+
+	if (p_blkno) {
+		if (ext_flags & OCFS2_EXT_UNWRITTEN) {
+			map->flags |= OCFS2_MAP_UNWRITTEN;
+		} else if (!(ext_flags & OCFS2_EXT_UNWRITTEN)) {
+			/* Treat the unwritten extent as a hole for zeroing purposes. */
+			map->flags |= OCFS2_MAP_MAPPED;
+		} else {
+			/* nothing to do */
+		}
+	}
+
+	if (!ocfs2_sparse_alloc(osb)) {
+		if (map->pblk == 0) {
+			err = -EIO;
+			mlog(ML_ERROR,
+			     "iblock = %llu p_blkno = %llu blkno=(%llu)\n",
+			     (unsigned long long)map->lblk,
+			     (unsigned long long)map->pblk,
+			     (unsigned long long)OCFS2_I(inode)->ip_blkno);
+			mlog(ML_ERROR, "Size %llu, clusters %u\n",
+			     (unsigned long long)i_size_read(inode),
+			     OCFS2_I(inode)->ip_clusters);
+			dump_stack();
+			goto bail;
+		}
+	}
+
+	past_eof = ocfs2_blocks_for_bytes(inode->i_sb, i_size_read(inode));
+
+	if (create && (map->lblk >= past_eof))
+		map->flags |= OCFS2_MAP_NEW;
+
+bail:
+	if (err < 0)
+		return -EIO;
+	else
+		return map->len;
+}
+
 int ocfs2_get_block(struct inode *inode, sector_t iblock,
 		    struct buffer_head *bh_result, int create)
 {
diff --git a/fs/ocfs2/aops.h b/fs/ocfs2/aops.h
index 114efc9111e4..8dd6edd7c1a1 100644
--- a/fs/ocfs2/aops.h
+++ b/fs/ocfs2/aops.h
@@ -42,6 +42,8 @@ int ocfs2_size_fits_inline_data(struct buffer_head *di_bh, u64 new_size);
 
 int ocfs2_get_block(struct inode *inode, sector_t iblock,
 		    struct buffer_head *bh_result, int create);
+int ocfs2_map_blocks(struct inode *inode, struct ocfs2_map_block *map,
+		    int flags);
 /* all ocfs2_dio_end_io()'s fault */
 #define ocfs2_iocb_is_rw_locked(iocb) \
 	test_bit(0, (unsigned long *)&iocb->private)
diff --git a/fs/ocfs2/buffer_head_io.c b/fs/ocfs2/buffer_head_io.c
index 7bfe377af2df..493f2209cca5 100644
--- a/fs/ocfs2/buffer_head_io.c
+++ b/fs/ocfs2/buffer_head_io.c
@@ -23,18 +23,6 @@
 #include "buffer_head_io.h"
 #include "ocfs2_trace.h"
 
-/*
- * Bits on bh->b_state used by ocfs2.
- *
- * These MUST be after the JBD2 bits.  Hence, we use BH_JBDPrivateStart.
- */
-enum ocfs2_state_bits {
-	BH_NeedsValidate = BH_JBDPrivateStart,
-};
-
-/* Expand the magic b_state functions */
-BUFFER_FNS(NeedsValidate, needs_validate);
-
 int ocfs2_write_block(struct ocfs2_super *osb, struct buffer_head *bh,
 		      struct ocfs2_caching_info *ci)
 {
diff --git a/fs/ocfs2/ocfs2.h b/fs/ocfs2/ocfs2.h
index 62cad6522c7a..095f7ae5dded 100644
--- a/fs/ocfs2/ocfs2.h
+++ b/fs/ocfs2/ocfs2.h
@@ -509,7 +509,50 @@ struct ocfs2_super
 	struct ocfs2_filecheck_sysfs_entry osb_fc_ent;
 };
 
-#define OCFS2_SB(sb)	    ((struct ocfs2_super *)(sb)->s_fs_info)
+/*
+ * Bits on bh->b_state used by ocfs2.
+ *
+ * These MUST be after the JBD2 bits.  Hence, we use BH_JBDPrivateStart.
+ */
+enum ocfs2_state_bits {
+	BH_NeedsValidate = BH_JBDPrivateStart,
+};
+
+/* Expand the magic b_state functions */
+BUFFER_FNS(NeedsValidate, needs_validate);
+
+/*
+ * Logical to physical block mapping, used by ocfs2_map_blocks()
+ *
+ * This structure is used to pass requests into ocfs2_map_blocks() as
+ * well as to store the information returned by ocfs2_map_blocks().  It
+ * takes less room on the stack than a struct buffer_head.
+ */
+#define OCFS2_MAP_NEW			BIT(BH_New)
+#define OCFS2_MAP_MAPPED		BIT(BH_Mapped)
+#define OCFS2_MAP_UNWRITTEN		BIT(BH_Unwritten)
+/* useless? #define OCFS2_MAP_BOUNDARY		BIT(BH_Boundary) */
+/* useless? #define OCFS2_MAP_DELAYED		BIT(BH_Delay) */
+#define OCFS2_MAP_DIRTY			BIT(BH_Dirty)
+#define OCFS2_MAP_UPTODATE		BIT(BH_Uptodate)
+#define OCFS2_MAP_NEEDS_VALIDATE	BIT(BH_NeedsValidate)
+#define OCFS2_MAP_DEFER_COMPLETION	BIT(BH_Defer_Completion)
+#define OCFS2_MAP_FLAGS		(OCFS2_MAP_NEW | OCFS2_MAP_MAPPED |\
+				 OCFS2_MAP_DIRTY | OCFS2_MAP_UPTODATE |\
+				 OCFS2_MAP_NEEDS_VALIDATE |\
+				 OCFS2_MAP_DEFER_COMPLETION)
+
+struct ocfs2_map_block {
+	u64 pblk; /* physical block# */
+	u64 lblk; /* logical block# */
+	u64 len;  /* number of block */
+	unsigned int flags;
+};
+
+/* Flags used by ocfs2_map_blocks() */
+#define OCFS2_GET_BLOCKS_CREATE	(0x0001)
+
+#define OCFS2_SB(sb)	((struct ocfs2_super *)(sb)->s_fs_info)
 
 /* Useful typedef for passing around journal access functions */
 typedef int (*ocfs2_journal_access_func)(handle_t *handle,
-- 
2.54.0