nilfs2: add iomap operations for direct I/O reads

Add iomap.c and iomap.h with a read-only nilfs_iomap_ops,
wrapping the existing nilfs_bmap_lookup_contig() lookup
to report a mapped range, a real hole, or an EOF-clamped
range to iomap core.

NILFS2 is a log-structured, copy-on-write filesystem:
newly allocated blocks are only given a real disk address
when the segment constructor writes them out as part of a log,
which walks buffer_head lists directly and is not integrated
with the generic address_space writeback path. Because of that,
only the read-only side of the mapping is added - buffered writes,
writeback, and mmap's ->page_mkwrite() will stay on the existing
buffer_head based path (nilfs_get_block(), nilfs_write_begin/end(),
nilfs_writepages(), nilfs_dirty_folio(), block_page_mkwrite()).

Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>
Link: https://patch.msgid.link/20260724222745.2107464-2-slava@dubeyko.com
Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
cc: linux-nilfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

3 files changed, 85 insertions, 1 deletions

diff --git a/fs/nilfs2/Makefile b/fs/nilfs2/Makefile
index 43b60b8a4d07..516e6b85a03c 100644
--- a/fs/nilfs2/Makefile
+++ b/fs/nilfs2/Makefile
@@ -3,4 +3,4 @@ obj-$(CONFIG_NILFS2_FS) += nilfs2.o
 nilfs2-y := inode.o file.o dir.o super.o namei.o page.o mdt.o \
 	btnode.o bmap.o btree.o direct.o dat.o recovery.o \
 	the_nilfs.o segbuf.o segment.o cpfile.o sufile.o \
-	ifile.o alloc.o gcinode.o ioctl.o sysfs.o
+	ifile.o alloc.o gcinode.o ioctl.o sysfs.o iomap.o
diff --git a/fs/nilfs2/iomap.c b/fs/nilfs2/iomap.c
new file mode 100644
index 000000000000..3ae3bf6ed368
--- /dev/null
+++ b/fs/nilfs2/iomap.c
@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * NILFS iomap support implementation.
+ *
+ * Written by Viacheslav Dubeyko.
+ */
+
+#include <linux/iomap.h>
+#include <linux/pagemap.h>
+#include "nilfs.h"
+#include "mdt.h"
+#include "iomap.h"
+
+static int nilfs_iomap_begin(struct inode *inode, loff_t offset,
+			     loff_t length, unsigned int flags,
+			     struct iomap *iomap, struct iomap *srcmap)
+{
+	struct the_nilfs *nilfs = inode->i_sb->s_fs_info;
+	struct nilfs_inode_info *ii = NILFS_I(inode);
+	sector_t blkoff = offset >> inode->i_blkbits;
+	unsigned int maxblocks;
+	__u64 blknum = 0;
+	int ret;
+
+	/* Completely beyond EOF. Treat as hole */
+	if (i_size_read(inode) <= offset) {
+		iomap->type = IOMAP_HOLE;
+		iomap->addr = IOMAP_NULL_ADDR;
+		iomap->offset = offset;
+		iomap->length = length;
+		return 0;
+	}
+
+	/* Clamp length if the requested range goes beyond i_size */
+	if (offset + length > i_size_read(inode)) {
+		loff_t i_size = i_size_read(inode);
+		unsigned int blocksize = i_blocksize(inode);
+
+		length = round_up(i_size, blocksize) - offset;
+	}
+
+	maxblocks = min_t(loff_t, length >> inode->i_blkbits, INT_MAX);
+	if (maxblocks == 0)
+		maxblocks = 1;
+
+	down_read(&NILFS_MDT(nilfs->ns_dat)->mi_sem);
+	ret = nilfs_bmap_lookup_contig(ii->i_bmap, blkoff, &blknum, maxblocks);
+	up_read(&NILFS_MDT(nilfs->ns_dat)->mi_sem);
+
+	if (ret == -ENOENT) {
+		iomap->type = IOMAP_HOLE;
+		iomap->addr = IOMAP_NULL_ADDR;
+		iomap->offset = offset;
+		iomap->length = min_t(loff_t, length, i_blocksize(inode));
+		return 0;
+	} else if (ret < 0)
+		return ret;
+
+	iomap->bdev = inode->i_sb->s_bdev;
+	iomap->offset = offset;
+	iomap->length = min_t(loff_t, length, (loff_t)ret << inode->i_blkbits);
+	iomap->addr = (loff_t)blknum << inode->i_blkbits;
+	iomap->type = IOMAP_MAPPED;
+	iomap->flags = IOMAP_F_MERGED;
+
+	return 0;
+}
+
+const struct iomap_ops nilfs_iomap_ops = {
+	.iomap_begin = nilfs_iomap_begin,
+};
diff --git a/fs/nilfs2/iomap.h b/fs/nilfs2/iomap.h
new file mode 100644
index 000000000000..adef3e22346d
--- /dev/null
+++ b/fs/nilfs2/iomap.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * NILFS iomap support declarations.
+ *
+ * Written by Viacheslav Dubeyko.
+ */
+
+#ifndef _NILFS_IOMAP_H
+#define _NILFS_IOMAP_H
+
+extern const struct iomap_ops nilfs_iomap_ops;
+
+#endif /* _NILFS_IOMAP_H */